Ultrasonic detector and method for ultrasonic detection

ABSTRACT

While a transmitting transducer ( 2   a ) for transmitting an ultrasonic wave and a receiving transducer ( 2   b ) for receiving an ultrasonic wave are moved within a predetermined circular region ( 7 ) on a surface of a material being measured, ultrasonic waves are transmitted and received 10,000 times. Then, arithmetic averaging is performed every time an ultrasonic wave is received, on the ultrasonic wave and ultrasonic waves that have been received until then. For example, the aforementioned predetermined frequency is given by ((n±(½))×(10 6 ×v/ΔL))(Hz), where ΔL is a variation in distance between the transmitting transducer and the receiving transducer, v is a transmission velocity of an ultrasonic wave transmitting in a material being detected, and n is a natural number. Consequently, it is possible to detect, with high accuracy, the thickness of a concrete material having a narrow width and a thick thickness, the thickness of the covering of a reinforcing bar and the diameter thereof, the depth of a crack and the like.

TECHNICAL FIELD

The present invention relates to ultrasonic detection apparatuses to beemployed such as for detecting internal defects or the like of concretematerials by means of ultrasonic waves and an ultrasonic detectionmethod that employs the apparatus. More particularly, the presentinvention relates to an ultrasonic detection apparatus which providesaccurate and high-speed detection of reinforcing bars arranged inside aconcrete material, the depth of a crack, the thickness of concrete, gapsand the like, and to an ultrasonic detection method that employs theapparatus.

BACKGROUND OF ART

A concrete material is a composite structure of cement and coarseaggregates of 1 to 3 mm in diameter. Ultrasonic waves traveling througha concrete material are scattered while being reflected, refracted, andchanged in mode repeatedly at the interface between the coarse aggregateand the cement.

This causes readily the ultrasonic waves to be diffused in the concretematerial and significantly attenuated in strength in the orientationdirection of the ultrasonic waves. The level of the attenuation would beacceleratingly increased as the ultrasonic waves have higherfrequencies.

In addition, when longitudinal or transverse ultrasonic waves are inputinto a concrete material from a surface thereof, longitudinal ortransverse ultrasonic waves and direct waves, each having a relativelylarge amount of energy, coexist with the longitudinal or transverseultrasonic waves input to the inside of the concrete material. Inaddition, surface waves having a large amount of energy are generated atthe surface of the concrete material.

These phenomena have conventionally made it difficult to detect theinside such as of a concrete material or a porous material by means ofultrasonic waves.

However, recent years have seen an improvement of internal detectionmethods employing ultrasonic waves. Thus, in some cases, with variousconditions being satisfied, it is possible to measure the thickness of aconcrete plate or detect gaps or the like therein within a detectiondepth range of about 20 to 50 cm. The conditions of the detection areshown below.

First, it is necessary to use ultrasonic wave transmitting and receivingtransducers having a resonant frequency of about 100 to 500 kHz.Secondly, it is necessary to use transducers having an oscillator aslarge as about 50 to 70 mm in diameter. Thirdly, it is necessary toapply a stepped voltage to a ceramic oscillator or the like in thetransducer instead of the pulsed voltage, which has been conventionallyemployed.

FIG. 68(a) is a graph showing a pulsed voltage, (b) being a graphshowing the spectrum of the pulsed voltage, and (c) being a graphshowing a time series waveform of the pulsed voltage. On the other hand,FIG. 69(a) is a graph showing a stepped voltage, (b) being a graphshowing the spectrum of the stepped voltage, and (c) being a graphshowing a time series waveform of the stepped voltage. The graphsrepresent pulsed and stepped voltages having values of 50 to 500V.Differences are found in the spectrum and time series waveform betweenthe pulsed and stepped voltages. Incidentally, the peak frequencies ofFIGS. 68(b) and 69(b) are resonant frequencies of oscillators, whileFIGS. 68(c) and 69(c) show time series transmit ultrasonic waves.

Now, a conventional method for measuring a concrete material will beexplained in which the stepped voltage shown in FIG. 69(a) is applied tothe concrete material by using an ultrasonic transducer having anoscillator 56 mm in diameter whose resonant frequency is 1 MHz. FIG. 70is a schematic view illustrating a concrete plate as a material to bedetected. The concrete plate 41 as a material to be detected has athickness of 20 cm and contains fine stones about 2 mm in diameter ascoarse aggregate. In addition, the concrete plate 41 has a relativelysmall number of bubbles therein. Furthermore, it should be understoodthat this measuring method works as a method for making a measurementwith one transducer, in which a transducer 42 functions as receiving andtransmitting transducers. FIG. 71 is a graph illustrating a reflectedwave obtained under the aforementioned conditions, with the horizontalaxis representing the time and the vertical axis representing theamplitude.

Referring to FIG. 71, a peak 43 a shows a longitudinal reflected wave 43from the bottom surface of the concrete plate. The peak 43 a isnoticeable, showing that it is possible to measure the thickness of theconcrete plate under the aforementioned conditions.

Suppose that like the concrete plate 41, the thickness is relativelythin when compared with the surface area. In this case, according tovarious types of measurement examples, since a corner-reflected wave 44from a corner portion and a reflected wave of a surface wave 45 aregenerally small in amplitude, it is made possible to measure thethickness of a plate as thick as about down to 50 cm under theaforementioned conditions.

However, for a concrete plate having been subjected to aging, it isoften difficult to confirm the generation of a reflected wave from thebottom surface thereof. Likewise, when a concrete plate is not a planarone, and great amounts of reflected waves from the corner portions andfrom surface waves are provided and lots of bubbles are contained in theconcrete plate, it is also difficult in many cases to confirm thegeneration of a reflected wave from the bottom surface.

For example, the following cases make it difficult to measure thickness.FIG. 72 is a view illustrating a concrete pillar or a material to bedetected, (a) being a schematic view thereof before being cut apart and(b) being a schematic view thereof after having been cut apart.

Here, such a concrete pillar 51 was made that has a side of length 30 cmand another side of length 50 cm in a cross section perpendicular to thelongitudinal direction. Inside the concrete pillar 51, there is presenta large number of bubbles about 1 to 10 mm in diameter. In addition,contained in the concrete pillar are 30 wt % of coarse aggregates havingdiameters greater than 5 mm and less than 1 cm, 40 wt % of coarseaggregates having diameters greater than 1 cm and less than 2 cm, and 40wt % of coarse aggregates having diameters greater than 2 cm. Inaddition, a concrete material 51 a having a height of 50 cm was cut fromthe concrete pillar 51.

Such a case is explained below in which a transducer 52 is placed at thecenter A of a plane having a width of 50 cm for measuring the thickness.FIG. 73 is a schematic view illustrating waves produced when thethickness is measured with the transducer 52 being placed at the centerA.

When longitudinal ultrasonic waves are input into the concrete material51 a from a surface thereof directly downwards with the transducer 52being placed at the center A, as shown in FIG. 73, a corner-reflectedwave 54, a direct wave 55, a surface wave 56, and a longitudinal wave 57low in strength as well as a reflected wave 53 from the bottom surfacereturn to the center A. Accordingly, the received wave at the center Ais a superimposed wave of the waves 53-57, making it difficult todetermine the peak of the reflected wave from the bottom surface asshown in FIG. 71.

Various types of oscillators were actually used for the application of astepped voltage of 500V for measurement, with the results beingillustrated. FIG. 74(a) is a graph illustrating a time series waveformobtained by a measurement with a transmitting transducer having anoscillator of resonant frequency 2.5 MHz and 20 mm in diameter, (b)being a graph illustrating a time series waveform obtained by ameasurement with a transmitting transducer having an oscillator ofresonant frequency 500 kHz and 40 mm in diameter, and (c) being a graphillustrating a time series waveform obtained by a measurement with atransmitting transducer having an oscillator of resonant frequency 500kHz and 70 mm in diameter. Incidentally, the receiving transduceremployed an oscillator having a resonant frequency of 2.5 MHz and adiameter of 20 mm. Referring to FIGS. 74(a) through (c), ultrasonicwaves are transmitted at time 104 μs on the horizontal axis. Forexample, time 205 μs in the figures shows that 101 μs have elapsed afterthe time of transmission.

For these measurements, a two-transducer method was employed in which atransmitting transducer and a receiving transducer are arrangedextremely close to each other. Referring to FIGS. 74(a) through (c), thetime shown by the dashed lines indicates the theoretical time ofgeneration of the reflected wave 53 from the bottom surface of theconcrete material 51 a. However, in these time series waveforms, it isimpossible to identify the time as the time of generation of thereflected wave 53. Therefore, in such cases, it is impossible to measurethe thickness of the concrete material 51 a.

The present invention was developed in view of such problems. It is anobject of the present invention to provide an ultrasonic detectionapparatus which can detect with accuracy the thickness of a concretematerial having a narrow width and a thick thickness, the thickness ofthe covering of a reinforcing bar and the diameter thereof, the depth ofa crack and the like, and a ultrasonic detection method that employs theapparatus.

DISCLOSURE OF THE INVENTION

A first ultrasonic detection apparatus according to the presentinvention is for allowing a transmitting transducer to transmit anultrasonic wave a plurality of times to analyze an ultrasonic wavereceived by a receiving transducer. The ultrasonic detection apparatuscomprises: an arithmetic averaging device which performs arithmeticaveraging a plurality of times per one detection every time anultrasonic wave is received, on the ultrasonic wave and ultrasonic waveshaving been received until then; and extracting means which extracts anultrasonic wave having a predetermined frequency as a center frequencyfrom received ultrasonic waves. The abovementioned predeterminedfrequency is given by ((n±(½))(10⁶×vΔL))(Hz), where ΔL is a variation indistance between the abovementioned transmitting transducer and theabovementioned receiving transducer, v is a transmission velocity of anultrasonic wave transmitting in a material being detected, and n is anatural number.

The present invention allows the arithmetic averaging device to performarithmetic averaging 1,000 times or more per one detection every time anultrasonic wave is received, on the ultrasonic wave and the ultrasonicwaves that have been received until then. This causes waves havingvariations in phase to gradually cancel out each other and only thosewaves having substantially no variation in phase to amplify each otherto remain. Accordingly, measurements carried out under the conditionswhich cause substantially no change in phase of a desired wave wouldmake it possible to detect, with high accuracy, the thickness of aconcrete material narrow in width and thick in thickness or the like.Furthermore, the arithmetic averaging device performs directly thearithmetic averaging, thereby reducing the amount of processing to beperformed by purpose-oriented software or the like and making itpossible to perform processing at high speeds. For example, suppose thatarithmetic averaging needs to be performed 10,000 times, in which thearithmetic averaging device performs arithmetic averaging up to 4,000times and the software performs subsequent arithmetic averaging. In thiscase, arithmetic means obtained by performing arithmetic averaging 4,000times, another 4,000 times, and further 2,000 times are processed by thearithmetic averaging device, and then the resulting values are processedby the software.

A second ultrasonic detection apparatus according to the presentinvention is for allowing a transmitting transducer to transmit anultrasonic wave a plurality of times to analyze an ultrasonic wavereceived by a receiving transducer. The ultrasonic detection apparatuscomprises an arithmetic averaging device which performs arithmeticaveraging a plurality of times per one detection every time anultrasonic wave obtained by applying a step function voltage to anoscillator is received, on the ultrasonic wave and ultrasonic waveshaving been received until then. The abovementioned predeterminedfrequency is given by ((n±(½))×(10⁶×v/ΔL))(Hz), where ΔL is a variationin distance between the abovementioned transmitting transducer and theabovementioned receiving transducer, v is a transmission velocity of anultrasonic wave transmitting in a material being detected, and n is anatural number.

A third ultrasonic detection apparatus according to the presentinvention is for allowing a transmitting transducer to transmit anultrasonic wave a plurality of times to analyze an ultrasonic wavereceived by a receiving transducer. The ultrasonic detection apparatuscomprises: an arithmetic averaging device for performing arithmeticaveraging a plurality of times per one detection, every time anultrasonic wave obtained by applying a step function voltage to anoscillator is received, on the ultrasonic wave and ultrasonic waveshaving been received until then; and extracting means which extracts anultrasonic wave having a predetermined frequency as a center frequencyfrom received ultrasonic waves. The abovementioned predeterminedfrequency is given by ((n±(½(10⁶×v/66 L))(Hz), where ΔL is a variationin distance between the abovementioned transmitting transducer and theabovementioned receiving transducer, v is a transmission velocity of anultrasonic wave transmitting in a material being detected, and n is anatural number.

A first method for detecting an ultrasonic wave according to the presentinvention comprises the steps of: transmitting and receiving anultrasonic wave a plurality of times while a transmitting transducer fortransmitting ultrasonic waves and a receiving transducer for receivingultrasonic waves are moved within a predetermined region on a surface ofa material being detected; performing arithmetic averaging every timethe ultrasonic wave is received, on the ultrasonic wave and ultrasonicwaves having been received until then; and extracting an ultrasonic wavehaving a predetermined frequency as a center frequency from ultrasonicwaves obtained by the arithmetic averaging. The abovementionedpredetermined frequency is given by ((n±(½))×(10⁶×v/ΔL))(Hz), where ΔLis a variation in distance between the abovementioned transmittingtransducer and the abovementioned receiving transducer, v is atransmission velocity of an ultrasonic wave transmitting in a materialbeing detected, and n is a natural number.

The present invention allows ultrasonic waves to be transmitted andreceived a plurality of times while a transmitting transducer fortransmitting an ultrasonic wave and a receiving transducer for receivingan ultrasonic wave are moved within a predetermined region on a surfaceof a material being detected, thereby causing a received wave havingvariations in phase and a received wave having no variation in phase toexist. In addition, arithmetic averaging is performed, every time anultrasonic wave is received, on the ultrasonic wave and ultrasonic wavesthat have been received until then. This makes it possible to allowreceived waves varied in phase to gradually vanish and only those wavesnot varied in phase to remain. This makes it possible to vanishunnecessary received waves to extract only desired received waves.

A second method for detecting an ultrasonic wave according to thepresent invention comprises the steps of: transmitting and receiving anultrasonic wave a plurality of times while a transmitting-receivingtransducer for transmitting and receiving ultrasonic waves is movedwithin a predetermined region on a surface of a material being detected;performing arithmetic averaging every time the ultrasonic wave isreceived, on the ultrasonic wave and ultrasonic waves having beenreceived until then; and extracting an ultrasonic wave having apredetermined frequency as a center frequency from ultrasonic wavesobtained by the arithmetic averaging. The abovementioned predeterminedfrequency is given by ((n±({fraction (1/2)}(10⁶×v/ΔL))(Hz), where ΔL isa variation in distance between the abovementioned transmittingtransducer and the abovementioned receiving transducer, v is atransmission velocity of an ultrasonic wave transmitting in a materialbeing detected, and n is a natural number.

A third method for detecting an ultrasonic wave according to the presentinvention comprises the step of repeating a predetermined number oftimes the steps of: transmitting and receiving an ultrasonic wave aplurality of times while a transmitting transducer for transmittingultrasonic waves and a receiving transducer for receiving ultrasonicwaves, evenly spaced apart from each other, are moved within apredetermined region on a surface of a material being detected;performing arithmetic averaging every time the ultrasonic wave isreceived, on the ultrasonic wave and ultrasonic waves having beenreceived until then; and varying a distance between the abovementionedtransmitting transducer and the abovementioned receiving transducer by apredetermined amount. The method further comprises the steps of;determining an arithmetic mean of ultrasonic waves obtained as resultsof the arithmetic averaging; and extracting an ultrasonic wave having apredetermined frequency as a center frequency from ultrasonic wavesobtained by the last arithmetic averaging. The abovementionedpredetermined frequency is given by ((n±(½(10⁶×v/ΔL))(Hz), where ΔL is avariation in distance between the abovementioned transmitting transducerand the abovementioned receiving transducer, v is a transmissionvelocity of an ultrasonic wave transmitting in a material beingdetected, and n is a natural number.

A fourth method for detecting an ultrasonic wave according to thepresent invention comprises the step of repeating the steps of:transmitting and receiving an ultrasonic wave a plurality of times whilea transmitting transducer and a receiving transducer are evenly spacedapart from each other, the transmitting transducer transmittingultrasonic waves by receiving an electrical signal to be output from atransmitting circuit, and the receiving transducer receiving ultrasonicwaves to input an electrical signal to a receiving circuit disposed in ahousing different from one for the transmitting circuit; performingarithmetic averaging every time the ultrasonic wave is received, on theultrasonic wave and ultrasonic waves having been received until then;extracting an ultrasonic wave having a predetermined frequency as acenter frequency from ultrasonic waves obtained by the arithmeticaveraging; and moving the transmitting transducer and the receivingtransducer on a surface of a material being detected. The abovementionedpredetermined frequency is given by ((n±({fraction(1/2)}))×(10⁶×v/ΔL))(Hz), where ΔL is a variation in distance betweenthe abovementioned transmitting transducer and the abovementionedreceiving transducer, v is a transmission velocity of an ultrasonic wavetransmitting in a material being detected, and n is a natural number.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an ultrasonic detection apparatusaccording to an embodiment of the present invention.

FIG. 2 is a schematic view illustrating an embodiment that employs aone-transducer method.

FIG. 3 is a schematic view illustrating the positional relationshipbetween a transmitting transducer and a receiving transducer in themethod according to a first embodiment method of the present invention.

FIGS. 4(a) through (c) are graphs illustrating time series waveformsresulted from a measurement according to the method of the firstembodiment.

FIG. 5 is a graph illustrating a time series waveform provided by themeasurement of the width of a concrete material 51 a.

FIG. 6 is a schematic view illustrating the positional relationshipbetween a transmitting transducer and a receiving transducer specifiedby a method according to a second embodiment method of the presentinvention.

FIG. 7 is a graph illustrating a time series waveform resulted from ameasurement according to the method of the second embodiment.

FIG. 8 is a view illustrating a material to be detected which isemployed in an example of measurement of the depth of a crack, (a) beinga perspective view, (b) being a plan view, (c) being a cross-sectionalview taken along line A—A of (b), and (d) being a cross-sectional viewtaken along line B—B of (b).

FIGS. 9(a) through (c) are graphs illustrating time series waveformsresulted from the measurement of the depth of a crack.

FIGS. 10(a) and (b) are schematic views illustrating a method for movingone transducer.

FIG. 11 is a cross-sectional view illustrating a material to be detectedwhich is employed in an embodiment of measuring the depth of a deformedreinforcing bar.

FIG. 12 is a graph illustrating a spectrum obtained when a measurementis made with transducers 33 a and 33 b remaining fixed at both sidesacross a fixed point C.

FIGS. 13(a) and (b) are graphs illustrating time series waveformsresulted from the measurement of the depth of a deformed reinforcingbar.

FIG. 14 is a graph illustrating spectra obtained by filtering of C₂^(n)·Y_(1,1).

FIGS. 15(a) and (b) are graphs illustrating time series waveformscorresponding to the spectra shown in FIG. 14.

FIG. 16 is a view illustrating a path of ultrasonic waves in a deformedreinforcing bar, (a) being a side view and (b) being a cross-sectionalview taken along line C—C of (a).

FIG. 17 is a schematic view illustrating ultrasonic waves transmittingin a circumferential direction of a circular reinforcing bar.

FIGS. 18(a) and (b) are graphs illustrating time series waveformsobtained from a concrete material having no cracks formed thereon.

FIG. 19 is a schematic view illustrating the transmission path ofultrasonic waves in a concrete material having no crack formed therein.

FIG. 20 is also a schematic view illustrating the transmission path ofultrasonic waves in a concrete material having no crack formed therein.

FIG. 21 is a schematic view illustrating a jig for a third embodiment ofthe present invention.

FIG. 22 is a view of an arithmetic mean y_(Dk)(t), an arithmetic meany_(Dk+1)(t), and their arithmetic mean when a given frequency componentis shifted by one cycle between the arithmetic mean y_(Dk)(t) and thearithmetic mean y_(Dk+1)(t), (a) being a schematic view illustrating thearithmetic mean y_(Dk)(t), (b) being a schematic view illustrating thearithmetic mean y_(Dk+1)(t), and (c) being a schematic view illustratingtheir arithmetic mean _(f)y_(ave)(t).

FIG. 23 is a view of an arithmetic mean y_(Dk)(t), an arithmetic meany_(Dk+1)(t), and their arithmetic mean when a given frequency componentis shifted by one-half cycle between the arithmetic mean y_(Dk)(t) andthe arithmetic mean y_(Dk+1)(t), (a) being a schematic view illustratingthe arithmetic mean y_(Dk)(t), (b) being a schematic view illustratingthe arithmetic mean y_(Dk+) ₁(t), and (c) being a schematic viewillustrating their arithmetic mean _(f/2)y_(ave)(t).

FIG. 24 is a schematic view illustrating the transmission of wavesproduced when a measurement is made between A′ and B′ shown in FIG.72(b) by employing a two-transducer method.

FIG. 25 is a view illustrating a wave obtained through arithmeticaveraging with two types of jigs being employed, (a) being a graphillustrating a Fourier spectrum and (b) being a graph illustrating atime series waveform.

FIG. 26 is a view illustrating a wave obtained with only jig D2 beingemployed, (a) being a graph illustrating a Fourier spectrum and (b)being a graph illustrating a time series waveform.

FIG. 27 is a view illustrating a wave obtained with the center frequencybeing at 130 kHz, (a) being a graph illustrating a Fourier spectrum and(b) being a graph illustrating a time series waveform.

FIG. 28 is a graph illustrating the relationship between a frequency fiand a normalized amplitude.

FIG. 29 is a cross-sectional view illustrating a concrete material inwhich a reinforcing bar as a subject to be detected is embedded.

FIG. 30 is a schematic view illustrating the transmission of a waveproduced upon detection of a deformed reinforcing bar 82 shown in FIG.73.

FIG. 31 is a view illustrating waves with a frequency component being2f_(B) and two types of jigs being employed, (a) being a schematic viewillustrating arithmetic mean y_(D1)(t) obtained with one transducerbeing placed at a short distance from the other, (b) being a schematicview illustrating arithmetic mean y_(D2)(t) obtained with one transducerbeing placed at a long distance from the other, and (c) being aschematic view illustrating their arithmetic mean y_(ave)(t).

FIG. 32 is a graph illustrating the Fourier spectrum of a frequencycomponent having a center frequency of 590 kHz used for gaining.

FIG. 33 is a schematic view illustrating a time series waveform at eachposition of measurement.

FIG. 34 is a schematic view illustrating distances between thetransducers of each jig.

FIG. 35 is a graph illustrating time series waveforms obtained when fourjigs are used.

FIG. 36(a) is a schematic view illustrating a generated wavecorresponding to peak 92, (b) being a schematic view illustrating agenerated wave corresponding to peak 93, (c) being a schematic viewillustrating generated waves corresponding to peaks 94 and 96, and (d)being a schematic view illustrating a generated wave corresponding topeak 64.

FIG. 37 is a graph illustrating a time series wave of a broadbandfrequency component gained from input ultrasonic waves with the centerfrequency being at 1100 kHz.

FIGS. 38(a) through (d) are schematic views illustrating waves obtainedwhen each jig is used and 38(e) is a schematic view illustrating theirarithmetic mean.

FIG. 39 is a view illustrating the procedure of arithmetic averagingaccording to equation 20.

FIG. 40 is a graph illustrating changes in spectrum.

FIG. 41 is a graph illustrating spectra _(B)a₅ and a₅.

FIG. 42 is a graph illustrating an arithmetic mean wave obtained whenboth transmitting and receiving transducers have an oscillator 40 mm indiameter whose resonant frequency of 500 kHz.

FIG. 43 is a graph illustrating a component wave gained by applyingequations 52 and 53 to the arithmetic mean wave of FIG. 42 with thefrequency shown in the following equation 55 being employed as thecenter frequency.

FIGS. 44(a) through (d) are schematic views illustrating various methodsfor scanning a transducer without using a measurement tool.

FIG. 45 is a cross-sectional view illustrating the typical shape of alongitudinal-wave transducer.

FIG. 46 is a schematic view illustrating the manner of transmission oflongitudinal ultrasonic waves input to a concrete material from asurface thereof directly downwards.

FIG. 47 is a graph illustrating the results of measurement by themethod, shown in FIG. 44(b), for scanning the model of concrete of FIG.72.

FIG. 48 is a graph illustrating the comparison between the spectra ofinterference waves interfering detection and waves of targets to bedetected such as plate thickness.

FIG. 49 is a graph illustrating the result of raising a wave or acomponent wave having a center frequency of 200 kHz of FIG. 42 to thethird power.

FIG. 50 is a graph illustrating the result of gaining a component wavewith the center frequency being at f_(D)=65 kHz.

FIG. 51 is a schematic cross-sectional view illustrating a model ofconcrete used for measurement.

FIG. 52 is a graph illustrating component waves gained at eachmeasurement point with the center frequency being at 190 kHz.

FIG. 53 is a graph illustrating an example obtained in the course ofshifting, by filtering, the center frequency employed for gaining a waveobtained by raising each component wave of FIG. 52 to the tenth power.

FIG. 54 is a graph illustrating a 200 kHz component wave provided bymeasurement 3.

FIG. 55 is a view illustrating an amplified component wave obtained at acenter frequency of 680 kHz reached after gradual sweeping of centerfrequencies towards higher frequencies.

FIG. 56 is a graph illustrating a component wave having a centerfrequency of 1 MHz.

FIG. 57 is a schematic view illustrating the transmission of variouswaves in a concrete material that has been subjected to aging.

FIG. 58 is a schematic view illustrating the path of critical refractedwaves.

FIG. 59 is a schematic view illustrating a method for detecting areinforcing bar in a concrete material on the surface of which cracksare formed.

FIG. 60 is a view illustrating a concrete material that has been leftfor five years dried after poured, (a) being a plan view thereof, (b)being a cross-sectional view taken along line D—D of (a), and (c) beinga cross-sectional view taken along E—E of (a).

FIG. 61 is a view illustrating waves received at measurement positionP28, (a) being a graph illustrating a case where no electrical noise nordisturbance has been eliminated and (b) being a graph illustrating acase where they have been eliminated.

FIG. 62 is a graph illustrating a Fourier spectrum with the centerfrequency being at 120 kHz.

FIG. 63 is a schematic view illustrating a time series wave obtained ateach measurement position when electrical noise or the like has beeneliminated.

FIG. 64 is a schematic view also illustrating a time series waveobtained at each measurement position when electrical noise and the likehave been eliminated, with the scale of FIG. 63 being changed.

FIG. 65 is a schematic view illustrating the transmission path ofrefracted waves at measurement positions P23 and P25.

FIG. 66 is a schematic view illustrating the order of generation atvarious paths.

FIG. 67(a) is a schematic view illustrating an arithmetic mean wavey_(A)(t) and (b) is a schematic view illustrating an arithmetic meanwave y_(B)(t).

FIG. 68(a) is a graph showing a pulsed voltage, (b) being a graphshowing the spectrum of the pulsed voltage, and (c) being a graphshowing a time series waveform of the pulsed voltage.

FIG. 69(a) is a graph showing a stepped voltage, (b) being a graphshowing the spectrum of the stepped voltage, and (c) being a graphshowing a time series waveform of the stepped voltage.

FIG. 70 is a schematic view illustrating a concrete plate as a materialto be detected.

FIG. 71 is a graph illustrating a reflected wave obtained under aprior-art measuring method.

FIG. 72 is a view illustrating a concrete pillar as a material to bedetected, (a) being a schematic view thereof before being cut apart and(b) being a schematic view thereof after having been cut apart.

FIG. 73 is a schematic view illustrating the transmission of a waveproduced when a transducer 52 is placed at center A for measurement ofthickness.

FIG. 74(a) through (c) are graphs illustrating time series waveformsresulted from a prior-art detection method.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, an ultrasonic detection apparatus according to embodiments of thepresent invention will be specifically explained below with reference tothe accompanying drawings.

FIG. 1 is a block diagram illustrating an ultrasonic detection apparatusaccording to an embodiment of the present invention.

The ultrasonic detection apparatus according to this embodiment of thepresent invention is provided with a stepped-voltage generator 1 forapplying a stepped voltage outwardly, a transmitting transducer 2 a forreceiving a stepped voltage applied by the stepped-voltage generator 1to transmit ultrasonic waves to a material to be detected, and areceiving transducer 2 b for receiving a reflected wave or the like fromthe interior of the material being detected. The ultrasonic detectionapparatus is also provided with an analyzer 4 for analyzing anelectrical signal provided by the receiving transducer 2 b, and adisplay device 5 for displaying the result of analysis provided by theanalyzer 4 and the waveform of a stepped voltage generated by thestepped-voltage generator 1.

The stepped-voltage generator 1 is provided with a stepped-voltagegenerator circuit 1 a for generating a stepped voltage, a current supplycircuit 1 b for supplying a current to the stepped-voltage generatorcircuit 1 a at controlled intervals, and a stepped-voltage drivingcircuit 1 c for feeding the stepped voltage to outside thestepped-voltage generator 1. Incidentally, for example, thestepped-voltage generator 1 generates a stepped voltage of 500V.

Furthermore, the analyzer 4 is provided with an amplifier circuit 4 aforamplifying an electrical signal received, a filter circuit 4 b forfiltering the amplified signal, an analog-to-digital converter (ADC) 4 cfor converting the filtered signal, a gate-array (arithmetic averagingdevice) 4 d, and a central processing unit (CPU) 4 e. The gate-array 4 dperforms arithmetic averaging on received waves every time a wave isreceived.

The analyzer 4 is further provided with a control circuit 4 f, whichcontrols the interval of current supply by the current supply circuit 1b, the range of amplification by the amplifier circuit 4 a, theoperation of the filter circuit 4 b, the recording interval and therecord data length of the ADC 4 c, and the number of times of additionby the gate-array 4 d. Incidentally, the control circuit 4 f iscontrolled by the CPU 4 e or an external notebook-type personalcomputer.

Incidentally, the ultrasonic detection apparatus according to thisembodiment shown in FIG. 1 is expressed with the two-transducer method;however, the one-transducer method may be employed. FIG. 2 is aschematic view illustrating an embodiment that employs theone-transducer method. In this case, one transducer 6 functions astransmitting and receiving transducers.

In addition, in detecting the thickness of a porous material such as aconcrete material or in detecting a reinforcing bar, ultrasonic wavesare significantly attenuated in the material and thus tend to providefeeble reflected waves from a detected target, which are included inreceived waves. For this reason, it is preferable to design anultrasonic detection apparatus so as to prevent electrical noise, oramong other things, standing noise from entering the ultrasonicdetection apparatus as much as possible.

In this regard, it is preferable to extremely reduce the voltagegenerator circuit 1 a and the voltage driving circuit 1 c in thestepped-voltage generator 1 to a size enough to be placed on-board,thereby incorporating the circuits not into the stepped-voltagegenerator 1 but into the transmitting transducer 2 a. This preventselectrical noise from entering the analyzer 4 at the time ofhigh-voltage drive.

Furthermore, waves that are received by the receiving transducer 2 b andconverted into an electrical signal (voltage) is feeble. For thisreason, the electrical noise entering between the transmittingtransducer 2 a and the analyzer 4 has a significant effect on the S/Nratio of the wave received.

In this regard, it is preferable to extremely reduce the amplifiercircuit 4 ain the analyzer 4 to a size enough to be placed on-board,thereby incorporating the circuit not into the analyzer 4 but into thereceiving transducer 2 b.

As described above, to obtain further improved measurement accuracy, itis preferable to incorporate the voltage generator circuit 1 a and thevoltage driving circuit 1 c into the transmitting transducer 2 a, andthe amplifier circuit 4 ainto the receiving transducer 2 b.

Now, described is a method according to a first embodiment of thepresent invention, which employs the aforementioned detection apparatus.The first embodiment method allows the pair of the transmittingtransducer 2 a and the receiving transducer 2 b to move arbitrarilywithin a circular region having a radius of 5 to 7 cm with center atcenter A in FIG. 72(b).

FIG. 3 is a schematic view illustrating the positional relationshipbetween a transmitting transducer and a receiving transducer in themethod according to the first embodiment of the present invention.Incidentally, in FIG. 3, the transmitting transducer 2 a having adiameter of 20 mm is hatched with solid lines, while the receivingtransducer 2 b having a diameter of 20 mm is hatched with dashed lines.In addition, a pair of transducers connected to each other with a chaindouble-dashed line indicates each transducer at a moment.

The first embodiment method employs the ultrasonic detection apparatusshown in FIG. 1 and performs measurements a large number of times, forexample, 10,000 times while allowing the position of each of thetransducers 2 a and 2 b to be varied within a predetermined circularregion 7 as shown in FIG. 3. At this time, the receive and transmitfaces of the transducers 2 a and 2 b each have to be always in contactwith a surface of the concrete material 51 a. Thus, it is necessary toapply oil or the like in advance as an ultrasonic wave transmittingmedium to measurement regions.

When the thickness of the concrete material 51 a is measured by thisembodiment method, the transmission distance of the corner-reflectedwave 54, the direct wave 55, the surface wave 56, and the feeblelongitudinal wave 57 low in strength varies as the position of each ofthe transducers 2 a and 2 b varies. On the other hand, if the distanceis sufficiently short between the transmitting transducer 2 a and thereceiving transducer 2 b, it is possible to assume that the path lengthof the reflected wave 53 from the bottom surface of the concretematerial 51 a as a detected target will not vary.

Thus, although the phase of the waves 54-57 varies every time theposition of each of the transducers 2 a and 2 b varies, the phase of thereflected wave 53 will not vary. Even with a variation in phase of thereflected wave 53, the amount of the variation is negligible inlow-frequency ultrasonic waves. Accordingly, as shown in FIG. 3,arithmetic averaging may be performed on a number of measurements afterreceive waves ω_(i)(t) have been obtained by making the measurementswhile the position of each of the transducers 2 a and 2 b is beingvaried, thereby reducing the amplitude contributed to by the waves 54-57and increasing the amplitude of the reflected wave 53 as a detectedtarget. Accordingly, it can be readily possible to recognize thegeneration of the reflected wave 53. Incidentally, though not shown inthe drawings, a number of scattered waves are also included in the wavesreceived. These scattered waves are generated in a non-standing mannerwith respect to the position of the transducer, thereby allowing thearithmetic averaging to eliminate the scattered waves. M-time arithmeticaveraging is expressed by the following equation 1. $\begin{matrix}{{y(t)} = {\frac{1}{m}{\sum\limits_{i = 1}^{m}{\omega_{i}(t)}}}} & (1)\end{matrix}$

Incidentally, about 10 to 1000 times of measurements are not enough tosufficiently reduce the amplitude of the waves 54-57. Thus, it isdifficult to recognize the generation of the reflected wave 53. Manymeasurements show that it is necessary to make about 1,000 times or moreof measurements. Thus, it is necessary to measure the thickness such asof a concrete pillar or a beam material by performing arithmeticaveraging on such an enormous number of measurement waves, and thiscalculation is preferably carried out as fast as possible.

In this regard, this exemplary method employs the gate-array 4 d toperform the aforementioned arithmetic averaging by the control of theCPU 4 e or an external notebook-type personal computer. In this case,the stepped-voltage generator 1 is allowed to apply a stepped voltage tothe transmitting transducer at any interval of 1.5-10 ms intervals.Then, each of the transducers 2 a and 2 b is moved to perform arithmeticaveraging every time a wave is received while the stepped voltage isapplied, for example, 10,000 times.

Incidentally, for example, the time required to perform arithmeticaveraging 10,000 times during the application of the stepped voltage at1.5 ms intervals is only about 15 seconds, which is sufficientlypractical. In addition, it is more preferable to perform the arithmeticaveraging in a shorter time. Accordingly, every predetermined number oftimes, for example, 1,000 times, a waveform indicative of the arithmeticmean at that point may be displayed on the display device 5 to allow theoperator to recognize the generation of a reflected wave from thedetected target. The operator may be thereby allowed to determine toterminate the measurement and the calculation of the arithmeticaveraging.

Now, explained is the measurement result obtained by performingarithmetic averaging 10,000 times using a transmitting transducer havinga resonant frequency of 2.5 MHz or 500 kHz. Here, the oscillator of thetransmitting transducer is 20 mm in diameter for a resonant frequency of2.5 MHz and is 40 mm or 70 mm for a resonant frequency of 500 kHz, whilethe oscillator of the receiving transducer is 20 mm in diameter foreither cases with a resonant frequency of 2.5 MHz. In addition, themagnitude of the stepped voltage is 500V.

FIG. 4 is a view illustrating the results of measurements according tothe first exemplary method. FIG. 4(a) is a graph illustrating a timeseries waveform provided by a measurement using a transmittingtransducer having a resonant frequency of 2.5 MHz and a diameter of 20mm. FIG. 4(b) is a graph illustrating a time series waveform provided bya measurement using a transmitting transducer having a resonantfrequency of 500 kHz and a diameter of 40 mm. FIG. 4(c) is a graphillustrating a time series waveform provided by a measurement using atransmitting transducer having a resonant frequency of 500 kHz and adiameter of 70 mm. Incidentally, in FIGS. 4(a) through (c), ultrasonicwaves are transmitted at time 104 μs on the horizontal axis. Forexample, time 205 μs in the figures shows that 101 μs have elapsed afterthe time of transmission.

In FIGS. 4(a) through (b), the time indicated by the dashed lines showsthe time of generation of the reflected wave 53 from the center B of thebottom face of the concrete material 51 a, and the peak of alongitudinal reflected wave emerges distinctly at that time. Referringto FIG. 4(a), a peak indicative of the reflected wave appears at thetime after a lapse of 142.3 μs from the transmit time. In FIGS. 4(b) and(c), a peak indicative of the reflected wave appears at the time after alapse of 141.3 μs from the transmit time. Thus, the thickness of theconcrete material according to the former is expressed by the followingequation 2, while the thickness of the concrete material according tothe latter is expressed by the following equation 3. Here, thepropagation velocity of a longitudinal ultrasonic wave through theconcrete material of this model is assumed to be 4.3 mm/μs.$\begin{matrix}{{\frac{1}{2} \times 142.3 \times 4.3 \times 10^{- 1}} = {30.6\quad ({cm})}} & (2) \\{{\frac{1}{2} \times 141.3 \times 4.3 \times 10^{- 1}} = {30.4\quad ({cm})}} & (3)\end{matrix}$

As shown above, the latter employing a low-frequency ultrasonic waveprovided a closer value to the actual value of 30 cm, and it was alsopossible to make a measurement with an error of about 2% even in theformer employing an oscillator having a small radius to transmit acomparatively high-frequency ultrasonic wave.

Incidentally, in the first embodiment method, the transmittingtransducer and the receiving transducer are placed in close proximity toeach other for measurement; however, the one-transducer method may beemployed in which one transducer 6, as shown in FIG. 2, functions astransmitting and receiving transducers. In this case, for a measurementof the thickness of a concrete material, the path length of thereflected wave from the bottom face thereof would not vary and thus itis easily possible to identify the generation of a reflected wave or adetected target, like in the case where the two-transducer method isemployed.

However, in some cases, it is difficult to identify a reflected waveeven by the first embodiment method according to the present invention,described in the foregoing. For example, in the concrete material shownin FIG. 72(b), it is difficult to identify the reflected wave or adetected target when a transducer 52′ is placed at center A′ of asurface 30 cm in width to measure the thickness according to the firstexemplary method.

FIG. 5 is a graph illustrating a time series waveform provided by themeasurement of the width (50 cm) of the concrete material 51 a.Incidentally, this measurement employed a transmitting transducer havingan oscillator 40 mm in diameter whose resonant frequency is 500 kHz anda receiving transducer having an oscillator 20 mm in diameter whoseresonant frequency is 2.5 MHz, with the magnitude of a stepped voltagebeing 500V. In addition, the number of times of arithmetic averaging was10,000 times. That is, it was under the same conditions as those of themeasurement in FIG. 4(b). Incidentally, referring to FIG. 5, ultrasonicwaves are transmitted at time 104 μs on the horizontal axis. Forexample, time 205 μs in the figure shows that 101 μs have elapsed afterthe time of transmission.

In FIG. 5, the time indicated by the dashed line is the theoretical timeof generation of the reflected wave 53′ from the center B′ of a side ofthe concrete material 51 a; however, unlike FIG. 4(b), it is difficultto identify the time as the time of generation of the reflected wave 53′in this time series waveform.

This is because, in the measurement at the center A and at the center B,the width of the surface on which the transducer is placed is 50 cm forthe former and 30 cm for the latter, while the thickness to be measuredis 30 cm for the former and 50 cm for the latter. As the width of thesurface on which the transducer is placed is reduced or the thickness tobe measured is increased, the energy of a direct wave, a surface wave,and a feeble longitudinal wave on a surface of the concrete becomesrelatively large, making it easier to superimpose these waves upon areflected wave to be detected. Furthermore, a corner-reflected wave anda reflected wave from the bottom face are received substantially at thesame time. For this reason, even when measurement is made using the samedetection apparatus by the same method, measurable and immeasurablewaves are produced.

In this regard, a second embodiment method of the present inventionallows the transmitting transducer and the receiving transducer to movewithin predetermined regions different from each other.

FIG. 6 is a schematic view illustrating the positional relationshipbetween a transmitting transducer and a receiving transducer in thesecond embodiment method of the present invention. Incidentally,referring to FIG. 6, a pair of transducers connected to each other by achain double-dashed line indicates each of the transducers at a moment.In FIG. 6, a transmitting transducer 12 a is greater than a receivingtransducer 12 b in diameter; however, they may be equal to each other orthe transmitting transducer 12 a may be smaller than the receivingtransducer 12 b in diameter.

As shown in FIG. 6, the second embodiment method allows an extremelylarge number of measurements, for example, 10,000 times of measurementsto be made while allowing the position of the transmitting transducer 12a to be continuously varied within an elliptical region 11 a and theposition of the receiving transducer 12 b to be continuously variedwithin an elliptical region 11 b. Incidentally, for example, thedistance L between the centers of the elliptical region 11 a and theelliptical region 11 b is 15 cm. Here, like in the first embodimentmethod, it is necessary to apply oil or the like in advance as anultrasonic wave transmitting medium to a surface of a material beingdetected. Then, like in the first embodiment method, using the detectionapparatus shown in FIG. 1, received waves are recorded every time astepped voltage is applied to the transmitting transducer, allowing thegate-array 4 d to go automatically perform the calculation of arithmeticaveraging.

Illustrated are the results actually provided by the use of thetransducers and application of a 500V stepped voltage for measurementaccording to the second embodiment. FIG. 7 is a graph illustrating atime series waveform resulted from a measurement according to the secondembodiment method. Incidentally, referring to FIG. 7, ultrasonic wavesare transmitted at time 104 μs on the horizontal axis. For example, time205 μs in the figure shows that 101 μs have elapsed after the time oftransmission.

In FIG. 7, the time indicated by the dashed line shows the time ofgeneration of the reflected wave 53′ from the center B′ of a side of theconcrete material 51 a, and the peak of a longitudinal reflected waveemerges distinctly at that time. In addition, a peak indicative of thereflected wave 53′ appears at the time after a lapse of 237.3 μs fromthe transmit time. Thus, the width of the concrete material obtainedfrom these peaks is 51.0 cm. Accordingly, it is possible to makemeasurement with an error of about 2%.

Incidentally, in the second embodiment, an elliptical region is employedin which the position of the transmitting transducer or the receivingtransducer varies; however, the region may be circular or rectangular.Nevertheless, in the case where the transducer is placed on a surfacereduced in width of one like the material being detected in thisembodiment, it is possible to make a very smooth measurement in anelliptical region or a rectangular region by employing the directionorthogonal to the width as the direction of the minor axis or shorterside. In addition, what type of region should be employed as the regioncan be determined by the detected target or the measuring method (theone-transducer method or the two-transducer method). Furthermore, for anelliptical region or a rectangular region, the direction of the materialbeing detected, in which the longitudinal direction of the region isdirected, can be determined in accordance with the shape of the materialand the direction of a reinforcing bar arranged therein or the like.

Incidentally, the measurement of the thickness of a concrete materialaccording to the first and second embodiment methods has been explainedin the foregoing; however, the present invention is also applicable tothe measurement such as of a gap inside a concrete material and thedepth of a crack or the detection of a reinforcing bar.

Now, an actual example of measurement of the depth of a crack will beexplained below. FIG. 8 is a view illustrating a material to be detectedwhich is employed in an example of measurement of the depth of a crack,(a) being a perspective view, (b) being a plan view, (c) being across-sectional view taken along line A—A of (b), and (d) being across-sectional view taken along line B—B of (b).

A concrete block 21 as a material being detected has the shape of arectangular solid having a thickness of 30 cm and the other two sides 50cm in length. Inside the block, a total of six through reinforcing bars22 having a diameter of 19 mm are embedded to a depth of 5 cm from thefront or reverse surface and spaced apart by 15 cm. In addition, thereis formed a crack 23 about 1 mm in width and 15 cm in depth.

For the measurement of the depth of the crack 23 in the concrete block21 mentioned above, two transducers 12 a, 12 b having an oscillator 20mm in diameter whose resonant frequency was 2.5 MHz were placed acrossthe crack 23 in between two through reinforcing bars 22. In addition, astepped voltage of 500V was employed and successively applied to thetransmitting transducer 12 a at 5 ms intervals. That is, ultrasonicwaves were input to the concrete block 21 from the surface thereofdirectly downwards at intervals of 5 ms. At this time, the transmittingtransducer 12 a and the receiving transducer 12 b were moved quickly atrandom within the regions 11 a, 11 b while each of the ultrasonic-wavetransmit and receive surfaces was being kept in contact with the surfaceof the concrete block 21 via the ultrasonic wave transmitting medium.Then, received ultrasonic waves were recorded for each of inputultrasonic waves and arithmetic averaging was performed by the detectionapparatus shown in FIG. 1.

FIG. 9 is a view illustrating the result of a measurement of the depthof a crack, (a) being a graph illustrating a time series waveform withno arithmetic averaging having been performed, (b) being a graphillustrating a time series waveform with arithmetic averaging havingbeen performed 1,000 times, and (c) being a graph illustrating a timeseries waveform with arithmetic averaging having been performed 10,000times. Incidentally, referring to FIGS. 9(a) through (c), ultrasonicwaves are transmitted at time 104 μs on the horizontal axis. Forexample, time 205 μs in the figures shows that 101 μs have elapsed afterthe time of transmission.

As shown in FIG. 9(a), with no arithmetic averaging having beenperformed, a waveform indicative of the generation of a wave passingthrough a through reinforcing bar 22 having a short transmissiondistance appears prior to the time indicated by the dashed line. Thismakes it difficult to identify the time of generation of a wave thatdetours around the bottom portion of the crack 23.

On the other hand, as shown in FIGS. 9(b) and (c), with arithmeticaveraging having been performed 1,000 times or 10,000 times, thewaveform indicative of the generation of a wave passing through thethrough reinforcing bar 22 is diminished, making it possible to readilyidentify the time (69.8 μsec) indicated by the dashed line as the timeof generation of the wave that detours around the bottom portion of thecrack 23. Suppose the propagation velocity of an ultrasonic wave is 4.3m/μs in the concrete block 21. Then, the depth of the crack 23 can bedetermined by the following equation 4. $\begin{matrix}{{\frac{1}{2} \times 4.3 \times 69.8 \times 10^{- 1}} = {15.0\quad ({cm})}} & (4)\end{matrix}$

As can be seen from above, the value that is perfectly consistent withthe actual value was obtained. Like the aforementioned measurement ofthickness, this is because it can be assumed that a detouring wave 24which detours around the bottom portion of the crack 23 will not have achange in its path length, even when the position of each of thetransducers 12 a and 12 b varies within each of the movement regions 11a and 11 b, due to their geometric relationship, whereas thetransmission distance of a wave 25 passing through the throughreinforcing bar 22 varies significantly to thereby cause its phase tosignificantly vary. This caused the time series wave indicative of thegeneration of the detouring wave 24 to increase in amplitude as thenumber of times of arithmetic averaging increased, thereby causing theamplitude of the wave 25 to disappear.

Incidentally, the time series waveform shown in FIG. 9 is of neither thereceived wave nor the very one provided by performing arithmeticaveraging on the received wave but the one provided by performing thefollowing processing on them.

First, letting an original measurement wave be y(t), filtering wasperformed twice in accordance with the following equation 5 and 6between time 0 and 409 μs. $\begin{matrix}{{y_{1}(t)} = \frac{{y(t)} - {y( {t - {\Delta \quad t}} )}}{2}} & (5) \\{{y_{2}(t)} = \frac{{y_{1}(t)} - {y_{1}( {t + {\Delta \quad t}} )}}{2}} & (6)\end{matrix}$

Furthermore, filtering was performed six times in accordance with thefollowing equations 7 to 2. $\begin{matrix}{{y_{3}(t)} = \frac{{y_{2}(t)} - {y_{2}( {t - {\Delta \quad t}} )}}{2}} & (7) \\{{y_{4}(t)} = \frac{{y_{3}(t)} - {y_{3}( {t + {\Delta \quad t}} )}}{2}} & (8) \\{{y_{5}(t)} = \frac{{y_{4}(t)} - {y_{4}( {t - {\Delta \quad t}} )}}{2}} & (9) \\{{y_{6}(t)} = \frac{{y_{5}(t)} - {y_{5}( {t + {\Delta \quad t}} )}}{2}} & (10) \\{{y_{7}(t)} = \frac{{y_{6}(t)} - {y_{6}( {t - {\Delta \quad t}} )}}{2}} & (11) \\{{y_{8}(t)} = \frac{{y_{7}(t)} - {y_{7}( {t + {\Delta \quad t}} )}}{2}} & (12)\end{matrix}$

where Δt is (10⁶/(2×f_(HL))) and f_(HL) is 625 kHz.

Then, y₈(t) was illustrated in FIG. 9 as the time series waveform. Suchfiltering obviates the necessity of the inverse fast Fourier transform(FFT), thereby shortening the time for analysis. In addition, thisprevents an error caused by the inverse FFT operation from entering thetime series waveform. Furthermore, the aforementioned subject to beanalyzed is the original measurement wave from time 0 to 409 μs;however, the time for analysis can be significantly reduced by filteringonly the original measurement wave from time t_(a) to t_(b) shown inFIG. 9(c) as a subject to be analyzed.

Incidentally, the waveforms shown in FIGS. 9(b) and (c) ere obtained byarbitrarily moving both of the transmitting transducer and the receivingtransducer within each of the movement regions 11 a and 11 b as shown inFIG. 8. However, measurement may be made with the position of any onetransducer being fixed and only the other being moved. FIGS. 10(a) and(b) are schematic views illustrating a method for moving one transducer.

As shown in FIG. 10(a), methods for moving only one transducer 61 b withthe other transducer 61 a being fixed include a method for providing anarithmetic averaged time series wave while the transducer 61 b is movedon a segment of a general circle with center at the transducer 61 a. Onthe other hand, as shown in FIG. 10(b), the transducer 61 b may bearbitrarily moved within a predetermined region. In this case, a personskilled in the method of operating the detection apparatus can recognizethe time of generation without performing arithmetic averaging with thegate-array. That is, since waves passing through right and left throughreinforcing bars 62 a and 62 b interfere with each other, the wavespassing through the through reinforcing bars 62 a and 62 b willsubstantially disappear. Thus, the time of generation of a wavedetouring around the bottom portion of a crack 63 can be recognized bydisplaying a time series waveform on a display device every time thetransducer 61 b is moved. However, since this method requires a highlyskilled experience to recognize the time of generation, it is necessaryto perform arithmetic averaging to readily obtain the time ofgeneration.

Now, an embodiment of measuring the depth and diameter of a deformedreinforcing bar embedded in a concrete material will be explained. FIG.11 is a cross-sectional view illustrating a material being detectedwhich is employed in the embodiment of measuring the depth and diameterof the deformed reinforcing bar.

In a concrete material 31 to be detected, a deformed reinforcing bar 32having a diameter of 19 mm is embedded to a depth of 50 mm from asurface thereof.

In the measurement of the depth of the deformed reinforcing bar 32 inthe concrete material 31 mentioned above, a transmitting transducer 33 aand a receiving transducer 33 b, each having a resonant frequency of 2.5MHz, were placed immediately above the deformed reinforcing bar 32 andspaced apart by 40 mm form each other. In addition, a stepped voltage of500V was applied to the transmitting transducer 33 a successively 1,000times at 2.5 ms intervals. That is, the time is measured for about 2.5s. Then, received ultrasonic waves were recorded for each of inputultrasonic waves and arithmetic averaging was performed with thedetection apparatus shown in FIG. 1. Subsequently, filtering wasperformed on the waveform obtained by the arithmetic averaging.

In this filtering, used as a filter was the function shown by thefollowing equation 13, which was obtained by multiplyingsin^(k)((π/2)×(f/f_(HL))) by cos^(n)((π/2)×(f/f_(HL))), and Y_(B) ofarithmetic averaging wave y_(B) (t)=Y_(B) 19 exp(iω_(y)t) was multipliedby this function. $\begin{matrix}{{\sin^{k}( {\frac{\pi}{2} \cdot \frac{f}{f_{HL}}} )} \cdot {\cos^{n}( {\frac{\pi}{2} \cdot \frac{f}{f_{HL}}} )}} & (13)\end{matrix}$

Hereinafter, sin((π/2)×(f/f_(HL))) is denoted as C₁ and cos((π/2)×(f/f_(HL))) is denoted as C₂.

FIG. 12 is a graph illustrating a spectrum obtained when a measurementis made with the transducers 33 a and 33 b remaining fixed at both sidesacross a fixed point C. Incidentally, the spectrum shown in FIG. 12derives from a wave having broadband (0 to 2.5 MHz) oscillationcomponents that have been subjected to a filtering of C₁ ⁶·C₂ ⁴·Y (wheref_(HL)=2.5 MHz) under the conditions of a 1,000-timearithmetic-averaging received wave, y(t)=Y·exp(iω_(y)t), and f_(HL)=2.5MHz.

In addition, FIG. 13 is a view illustrating the results of themeasurement of the depth of the deformed reinforcing bar, (a) being agraph illustrating a time series waveform obtained by making ameasurement with each of the transducers 33 a and 33 b remaining fixedat both sides across the fixed point C, and (b) being a graphillustrating a time series waveform obtained by making a measurementwhile each of the transducers 33 a and 33 b was being moved from point Cto point D. Incidentally, referring to FIGS. 13(a) and (b), ultrasonicwaves are transmitted at time 104 μs on the horizontal axis. Forexample, time 204 μs in the figure shows that 100 μs have elapsed afterthe time of transmission.

A longitudinal ultrasonic wave in the concrete material 31 shown in FIG.11 has a propagation velocity of 4.2 mm/μs, and the generation of areflected wave 34 from the upper end of the reinforcing bar would berecognized 25.6 μs after the transmission in accordance with thefollowing equation 14. $\begin{matrix}{{2 \times {\sqrt{( \frac{40}{2} )^{2} + 50^{2}}/4.2}} = {25.6\quad ({µs})}} & (14)\end{matrix}$

However, as shown in FIG. 13(a), for the measurement performed with eachof the transducers 33 a and 33 b remaining fixed, it is impossible toidentify the generation of the reflected wave 34 at time 25.6 μs shownby the dashed line.

On the other hand, as shown in FIG. 13(b), for a wave that has beenobtained at each transmission of an ultrasonic wave by performingarithmetic averaging 1,000 times while each of the transducers 33 a and33 b is moved linearly from point C to point D immediately above thedeformed reinforcing bar 32 at a generally constant velocity with thetransducers 33 a and 33 b being kept spaced apart by 40 mm, thegeneration of a large amplitude in the reflected wave 34 can berecognized at time 25.7 μs shown by the dashed line. Incidentally, anultrasonic wave transmitting medium was applied in advance to a surfaceof the concrete material 31 so that the transmit and receive face ofeach of the transducers 33 a and 33 b could tightly contact with thesurface.

The vertical axes of FIGS. 13(a) and (b) have the same scale. A numberof waves having a large amplitude, generated in FIG. 13(a), show thegeneration of a direct wave 35, a surface wave 37, a longitudinal wave(not shown), and scattered wave (not shown). Since the distance betweenthe transmitting transducer 33 a and the receiving transducer 33 b is asshort as 40 mm, the direct wave 35, the surface wave 37, thelongitudinal wave, and the scattered wave are provided with a largeamplitude, which last-for a long time. In addition, the reflected wave34 as a target to be detected is submerged among these waves.

Incidentally, the presence of coarse aggregates (fine stones ofdiameters about 1 to 2 cm) and bubbles about 1 to 2 mm in diameter nearthe position of measurement causes the amplitude and phase of the directwave 35 and the scattered wave to significantly vary depending on theposition of measurement. Accordingly, waves having different phasescancel out each other by performing arithmetic averaging on the receivedwaves while the transmitting transducer 33 a and the receivingtransducer 33 b are moved, thereby causing the direct wave 35 and thescattered wave to disappear as the number of times of additionincreases. At this time, with the distance between the transducers beingkept unchanged, the reflected wave 34 from the upper end of thereinforcing bar and a reflected wave 36 from the lower end would notvary in their path length and in their phase as well. For this reason,the amplitude of the reflected wave 34 increases relatively as thenumber of times of addition increases. Accordingly, as shown in FIG.13(b), the reflected wave 34 from the upper end of the reinforcing barbecome prominent by performing arithmetic averaging 1,000 times.

However, only the generation of the reflected wave 34 can be recognizedin FIG. 13(b), but the generation of the reflected wave 36 from thelower end of the reinforcing bar cannot be recognized. For detection ofa reinforcing bar or the like having a circular cross section, thereflected wave 36 from the lower end of the reinforcing bar contains anextremely small amount of oscillation components. However, many examplesof measurement show that other generated waves that provide informationfor measuring the diameter of a reinforcing bar can be gained as alow-frequency component wave. A method for gaining a low-frequencycomponent wave will be explained below.

First, a filtering of C₂ ^(n)·Y_(1,1) is carried out under theconditions of a time series wave, y,_(1,1)(t)=Y_(1,1) ·exp(iω_(y)t), andf_(HL)=2.5 MHz. In practice, the calculations shown in the followingequation 15 and 16 may be carried out. $\begin{matrix}{{y_{n,1}(t)} = {\frac{{y_{{n - 1},1}(t)} - {y_{{n - 1},1}( {t - {\Delta \quad t}} )}}{2}\quad ( {n\text{:}\quad {an}\quad {odd}\quad {number}} )}} & (15) \\{{y_{n,1}(t)} = {\frac{{y_{{n - 1},1}(t)} - {y_{{n - 1},1}( {t + {\Delta \quad t}} )}}{2}\quad ( {n\text{:}\quad {an}\quad {even}\quad {number}} )}} & (16)\end{matrix}$

where Δt is (10⁶/(2×f_(HL))) and f_(HL) is 2.5 MHz. Then, y_(n,1)(t) isdetermined and subjected to the fast Fourier transform to determine C₂^(n)·Y_(1,1).

FIG. 14 is a graph illustrating spectra obtained by a filtering of C₂^(n)·Y_(1,1). Referring to FIG. 14, a thin line represents a spectrumfor n=25 and a bold line represents a spectrum for n=55.

FIG. 15 is a view illustrating time series waveforms corresponding tothe spectra shown in FIG. 14, (a) being a graph for n=25 and (b) being agraph for n=55. The time series waveforms shown in FIGS. 15(a) and (b)are provided by raising actually obtained time series waves to thefourth power at each point in time. Incidentally, referring to FIGS.15(a) and (b), ultrasonic waves are transmitted at time 104 μs on thehorizontal axis. For example, time 205 μs in the figure shows that 101μs have elapsed after the time of transmission.

As shown in FIGS. 15(a) and (b), the time series wave for n=55 containsa larger number of low-frequency components than the time series wavefor n=25. In addition, in the time series ave for n=55, it is possibleto recognize a peak 37 indicative of the generation of a wave forproviding information about the diameter of the deformed reinforcing bar32, which cannot be recognized in the time series wave that contains alarge number of high-frequency components shown in FIGS. 13(a) and (b),at the time shown by the dashed line after a peak 34 a of the reflectedwave 34 from the upper end of the reinforcing bar.

However, the wave indicated by the peak 37 does not show the reflectedwave 36 from the lower end of the reinforcing bar. FIG. 16 is a viewillustrating a path of ultrasonic waves in a deformed reinforcing bar,(a) being a side view and (b) being a cross-sectional view taken alongline C—C of (a). As shown in FIGS. 16(a) and (b), a reinforcing bar orthe like, circular in cross section, provides the reflected wave 36extremely feeble in strength from the lower end thereof, making itdifficult to recognize the wave in a number of measurements withoutusing other special measuring methods.

On the other hand, a time series wave that has been gained with alow-frequency broadband of 150 to 500 kHz as shown in FIG. 14 would makeit possible to recognize the peak 37 a shown in FIG. 15(b) in anymeasurement embodiments of the same type.

In this regard, it is conceivable that, in a reinforcing bar embedded ina concrete material and having a circular cross section, a pipe, acircular gap, and the like, there exists an ultrasonic wave thattransmits in their circumferential direction. FIG. 17 is a schematicview illustrating ultrasonic waves transmitting in a circumferentialdirection of a circular reinforcing bar. As a result of actuallymeasuring about 50 times and examining a circular reinforcing bar 32 a,it was confirmed that the peak 37 was the generation of superimpositionof waves transmitting in the reinforcing bar and detouring around theconcrete material in the direction of the circumference of thereinforcing bar. Consequently, derived was the following equation 17 forcalculating the diameter of the reinforcing bar in accordance with thetime of generation of the reflected wave from the upper end of thereinforcing bar and the time of generation of a detouring wave thatdetours around the reinforcing bar or the like. $\begin{matrix}{d = {( {t_{1} - t_{2}} ) \times \frac{V_{P}}{\pi}}} & (17)\end{matrix}$

where d is the diameter of the reinforcing bar, t₁ is the time ofgeneration of the reflected wave from the upper end of the reinforcingbar, t₂ is the time of generation of the detouring wave that detoursaround the reinforcing bar, and V_(P) is the propagation velocity of alongitudinal ultrasonic wave in the iron material.

By substituting t₁=25.5 (μs), t₂=37.3 (μs), and V_(P)=5.9 (mm/μs)obtained from the aforementioned measurement, into equation 17, it isgiven that d=22 (mm). The actual deformed reinforcing bar 32 of diameter19 mm has the maximum diameter of 21.5 and the minimum diameter of 18mm. Thus, the diameter is generally measured with accuracy.

Incidentally, the equation 17 holds not only for a reinforcing barcircular in cross section but also for the aforementioned pipe andcircular gap.

In addition, on the surface of the concrete material 31 shown in FIG.11, a plurality of fine cracks thinner than a hair have been produced.In contrast, different waveforms would be obtained by the samemeasurement made on a concrete material on which such cracks are notformed. FIGS. 18(a) and (b) are graphs illustrating time serieswaveforms obtained from such a concrete material having no crack formedthereon.

As shown in FIG. 18(a), the time series wave obtained from the concretematerial having no cracks formed thereon is significantly different fromthat obtained from the concrete material having cracks formed thereon asshown in FIG. 13(b) ). FIGS. 19(a) through (c) and FIGS. 20(a) and (b)are schematic views illustrating the transmission path of ultrasonicwaves in a concrete material having no cracks formed therein. In FIG.18(a), a generation 71 a of a reflected wave 71 appears in an extremelystrong manner from a portion of the minimum diameter of a deformedreinforcing bar 77. In addition, a generation 72 a of a surface wave 72between the transducers and a generation 73 a of a reflected wave 73from a portion of the maximum diameter of the deformed reinforcing bar77 also appear strongly.

On the other hand, FIG. 18(b) is a view illustrating increasedgenerations 74 a (within the reinforcing bar) and 74 b (within theconcrete material) of a detouring wave 74 that detours around theperiphery of the reinforcing bar, and a generation 75 a of acorner-reflected wave 75 from an edge portion 7.5 cm apart, with thereflected waves 71 and 73 and the surface wave 72 being diminished inamplitude. As described above, it is also possible to amplify theamplitude of a desired wave. Incidentally, the distance between thetransducers is 40 mm and the deformed reinforcing bar 77 is embedded toa depth of 50 mm from a surface of the concrete material. In addition,the generation of a reflected wave 76 from the lower end of the deformedreinforcing bar 77 as shown in FIG. 19(c) was not recognized even inFIGS. 18(a) and (b). Furthermore, the time series waveform shown in FIG.18(a) is provided by raising an actually obtained time series wave tothe third power at each point in time, while the time series waveformshown in FIG. 18(b) is provided by raising the wave to the third powerin the same manner.

In general, there exist a number of unrecognizable fine cracks on asurface of a concrete material and the deterioration or the like of theconcrete material due to aging cannot be avoided. Accordingly, the timeseries waves as shown in FIGS. 18(a) and (b) can be obtained only byextremely good luck. Thus, as shown in FIG. 14 and FIGS. 15(a), (b), itis necessary to gain a low-frequency component wave by performing theaforementioned processing.

In the foregoing, it is shown that the thickness of a concrete materialand the depth of a crack thereof, and the thickness of a covering andthe diameter of a reinforcing bar can be measured. However, in somecases, a satisfactory measurement cannot be provided only by theaforementioned method depending on the condition of the concretematerial. This is because of the following six properties of a concretematerial. First, cement and stones (coarse aggregates) 1 to 3 cm indiameter are mixed and hardened into a concrete material, in whichultrasonic waves are scattered at the interface between the cement andthe coarse aggregates. Secondly, typical concrete materials include aninfinite number of bubbles 1 to 10 m in diameter therein and thesebubbles amplify the scattering phenomenon. Thirdly, the strength ofconcrete materials varies greatly ranging from 360 to 700 (kg/cm²)depending on the subject being constructed therewith, and thetransmission and attenuation properties of ultrasonic waves varysignificantly depending on this strength. Fourthly, there exist adeterioration phenomenon due to aging in concrete materials, and thetransmission and attenuation properties of ultrasonic waves vary greatlydepending on the level of the deterioration. Fifthly, occurrence of thescattering phenomenon in causes the shape of a subject being detectedsuch as a floor, pillar, beam or the like to have a significant effecton the waveform of received ultrasonic waves. For example, in pillarsand beams, a large number of waves or the so-called direct waves thatdetour around in the concrete material are produced, causing a reflectedwave or the like from the subject being detected to be buried therein.Sixthly, a number of fine wide-range cracks are generally formed on asurface of the concrete material, and such cracks make detectiondifficult in some cases.

For example, for a concrete material being detected that has been leftin adverse environments for 10 to 20 years, it is difficult to measurethe thickness of the concrete material.

In such a case, measurement may be carried out with the following jigbeing attached to the transmitting transducer and the receivingtransducer, thereby making it possible to make the measurement withaccuracy. This measurement with the jig is to be employed as a thirdembodiment. FIG. 21 is a schematic view illustrating the jig for thethird embodiment of the present invention.

The third embodiment is provided with k types of jigs D1, D2, D3, . . .Dk for keeping the distance unchanged between a transmitting transducerC1 and a receiving transducer C2. The distance between the transducersin the jig D1 is l₁, the distance between the transducers in the jig D2is l₂, the distance between the transducers in the jig D3 is l₃, and thedistance between the transducers in the jig Dk is l_(k). In addition,the relationship that 1_(k+1)−1_(k) =Δl (constant) holds for thedistances between the transducers.

In the detection method employing the third embodiment provided withsuch a jig, arithmetic averaging is performed on measured waves for thesame number of times (n) for each jig in the same manner as in theaforementioned detection method, and thereafter arithmetic averaging isfurther performed on these arithmetic mean waves. Letting the receivedwave at the jth measurement with the jig Di be ω_(Di,j) (t), thearithmetic averaging y_(D i) (t) with the jig Di is expressed by thefollowing equation 18. $\begin{matrix}{{y_{Di}(t)} = {\frac{1}{n}{\sum\limits_{j = 1}^{n}{\omega_{{Di},j}(t)}}}} & (18)\end{matrix}$

Then, the gate-array or the CPU incorporated into the detectionapparatus is allowed to perform the arithmetic averaging of thefollowing equation 19 or 20, thereby calculating the arithmetic meany_(ave)(t) in accordance with all measurements. Incidentally, thisarithmetic averaging may be carried out with an external notebook-typepersonal computer. $\begin{matrix}{{y_{ave}(t)} = {\frac{1}{n - 1}{\sum\limits_{k = 1}^{n - 1}{\frac{1}{2}( {{y_{Dk}(t)} + {y_{{Dk} + 1}(t)}} )}}}} & (19) \\{{y_{ave}(t)} = {\frac{1}{n}{\sum\limits_{k = 1}^{n}{y_{Dk}(t)}}}} & (20)\end{matrix}$

Now, an effect that is obtained by calculating such an arithmetic meanwill be described hereafter. FIG. 22 is a view of an arithmetic meany_(Dk)(t), an arithmetic mean y_(Dk+1)(t), and their arithmetic meanwhen a given frequency component is shifted by one cycle between thearithmetic mean y_(Dk)(t) and the arithmetic mean y_(Dk+1)(t), (a) beinga schematic view illustrating the arithmetic mean y_(Dk)(t), (b) being aschematic view illustrating the arithmetic mean y_(Dk+1)(t), and (c)being a schematic view illustrating their arithmetic mean_(f)y_(ave)(t). When a given frequency component is shifted by one cyclebetween the arithmetic mean y_(Dk)(t) and the arithmetic meany_(Dk+1)(t) as shown in FIGS. 22(a) and (b), the amplitude of the firstone cycle is half that of the arithmetic mean y_(Dk)(t), and theamplitude in the subsequent cycles is the same as that of the arithmeticmeans y_(Dk)(t) and y_(Dk+1)(t), as shown in FIG. 22(a). Here, it isassumed that the amplitude of each component wave of the arithmeticmeans y_(Dk)(t) and y_(Dk+1)(t) is 1.0.

FIG. 23 is a view of an arithmetic mean y_(Dk)(t), an arithmetic meany_(Dk+1)(t), and their arithmetic mean when a given frequency componentis shifted by one-half cycle between the arithmetic mean y_(Dk)(t) andthe arithmetic mean y_(Dk+1)(t), (a) being a schematic view illustratingthe arithmetic mean y_(Dk)(t), (b) being a schematic view illustratingthe arithmetic mean y_(Dk−1)(t), and (c) being a schematic viewillustrating their arithmetic mean _(f/2)y_(ave)(t) When a givenfrequency component is shifted by one-half cycle between the arithmeticmean y_(Dk)(t) and the arithmetic mean y_(Dk+1)(t) as shown in FIGS.23(a) and (b), the amplitude of the first one cycle is half that of thearithmetic mean y_(Dk)(t), and the amplitude in the subsequent cycles iszero, as shown in FIG. 23(a).

Incidentally, a frequency component wave has zero amplitude in thesecond and subsequent cycles not only at a frequency of (½)f, but such aphenomenon occurs to a frequency component of (n±½)×f(n: naturalnumber).

Now, take an example of measurement or the like of the thickness of aconcrete material. In two arithmetic mean waves measured with jigs forproviding different distances between the transducers by Δl, there existwaves in transmission paths that cause the time of reception such as ofreflected waves to significantly vary. FIG. 24 is a schematic viewillustrating the transmission of waves produced when a measurement ismade between A′ and B′ shown in FIG. 72(b) by employing thetwo-transducer method.

The time of reception of a reflected wave 201 from the bottom face ofthe concrete material 51 a hardly varies even when the distance betweenthe transducers varies. However, variations in distance between thetransducers would cause the time of reception of path waves 202, 203,and 204 to significantly vary as described above.

Letting the transmission velocity of an ultrasonic wave be v(mm/μs), thefollowing equation 21 holds for the difference Δt(μs) in time ofreception between the two waves. $\begin{matrix}{{\Delta \quad t} = \frac{\Delta \quad l}{v}} & (21)\end{matrix}$

In addition, the frequency f corresponding thereto and having anarithmetic mean y_(ave)(t) shown in FIGS. 22 and 30 is expressed by thefollowing equation 22. $\begin{matrix}{f = \frac{10^{6}}{\Delta \quad t}} & (22)\end{matrix}$

Moreover, by making use of an equivalent sound velocity between thetransducers A₁ and A₂, it is possible to determine a general value ofthe frequency f.

Letting the transmission velocity of a longitudinal ultrasonic wave in atypical concrete material be 4.0(mm/μs), the transmission velocity ofthe aforementioned path waves 202 to 204 would vary from 3 to 4 (mm/μs).Accordingly, as shown in FIGS. 22(a) and (b), in the arithmeticaveraging obtained by using the equation 19 or 20, the path waves 202 to204 would not be attenuated but last for a long time, assuming that thecomponent waves within the range of frequencies shown in the followingequation 23 are not attenuated. This causes the reflected wave 201 beingdetected to be buried in the path waves 202 to 204. $\begin{matrix}{f = {\frac{10^{6}}{\Delta \quad {t/3}} \sim \frac{10^{6}}{\Delta \quad {t/4}}}} & (23)\end{matrix}$

On the other hand, with a frequency component of (½)f, the path waves202 to 204 are significantly attenuated as shown in FIG. 23(c).Furthermore, the transmission length of the reflected wave 201 hardlyvaries due to its geometric relationship, or a shift in phase hardlyoccurs, thereby relatively amplifying the reflected wave 201.

As described above, using the aforementioned two jigs, by gaining thecomponent having a center frequency corresponding to one-half of thefrequency f, which is obtained by the equations 21 and 22 with respectto a variation Δl in distance between the transducers, from the waveobtained by the arithmetic averaging in accordance with the equation 19or 20, the reflected wave 201 would emerge from the path waves 202 to204 without being buried therein.

Now, the method for measuring the thickness of an actual concretematerial according to the third embodiment of the present invention andthe results of the measurement will be described below.

Here, employed were two oscillators, having a resonant frequency of 2.5MHz and a diameter of 20 mm, and the jig D1 for keeping the transducersspaced apart by 81 mm, and the jig D2 for keeping the transducers spacedapart by 108 mm, where a stepped voltage was applied. That is, Δl is 27mm. Then, as shown in FIG. 3, while the transducers were being movedwithin a predetermined region, arithmetic averaging was performed 4,000times for each of the jigs to measure the distance between Δland B′ ofFIG. 72(b). Thereafter, the arithmetic averaging shown by equation 20was performed and the resulting wave was gained to yield a componentwave having a center frequency of 65 kHz. FIG. 25 is a view illustratinga wave of y_(ave)(t) for this case, (a) being a graph illustrating aFourier spectrum and (b) being a graph illustrating a time serieswaveform. As shown by the dotted line in FIG. 25(b), the generation of areflected wave emerges distinctly from the bottom face.

On the other hand, with only the jig D2 and without the arithmeticaveraging shown by the equation 19 or 20, it was difficult to recognizethe generation of a reflected wave. FIG. 26 is a view illustrating awave obtained with only the jig D2, (a) being a graph illustrating aFourier spectrum and (b) being a graph illustrating a time serieswaveform. In this case, a component wave having a center frequency of 65kHz was also gained from a combined wave indicative of arithmeticaveraging performed on 4,000-time measurements. The time indicated bythe dotted line in FIG. 26(b) is the theoretical time for the generationof a reflected wave, which is difficult to determine.

Incidentally, in the aforementioned detection, a component wave of 65kHz was finally gained because of the following reasons. SubstitutingΔl=27 mm into equation 23 would give a frequency f of 110 to 150 kHz.One-half of this value is 55 to 75 kHz. Moreover, the center of thisrange is 65 kHz.

Now, the wave obtained not by using the one-halve value of the frequencybut by using the center frequency 130 kHz of the range is describedbelow. In this case, since path waves except for a reflected wave areamplified, the determination of the arithmetic mean in accordance withthe equation 20 would cause the reflected wave 201 to be buried in thepath waves 202 to 204 as described above. FIG. 27 is a view illustratinga wave obtained with the center frequency being at 130 kHz, (a) being agraph illustrating a Fourier spectrum and (b) being a graph illustratinga time series waveform. The time indicated by the dotted line in FIG.27(b) is the theoretical time for the generation of the reflected wave201, which is difficult to determine.

Incidentally, the reflected wave 201 allows a component wave of (n+½)×fto emerge, where the frequency of f is calculated in accordance with theequations 21 and 22 using a variation Δl in path length due to theaforementioned two jigs;

however, this never happens in practice because of the followingreasons. For the arithmetic mean of the component waves shown in FIGS.22(c) and 23(c), it was assumed that the component waves have not beenattenuated. In practice, as shown in FIG. 25, since a reflected wave orthe like consists of several waves, the reflected wave 201 is notamplified but caused to disappear when the frequency is greater thanthat determined in accordance with the equations 21 and 22. In addition,this phenomenon is acceleratingly amplified as the concrete materialbeing measured becomes thicker in thickness and the wave componentsbecome higher in frequency. Because of these reasons, reflected wavesare not allowed to emerge in some cases.

Now, an actual method for measuring the planar position of a reinforcingbar and the thickness of a covering thereof according to the thirdembodiment of the present invention and the results thereof will bedescribed below.

FIGS. 22 and 30 are views illustrating how to obtain the arithmetic meanof a given low-frequency wave of frequency f, the phase of which isshifted by one cycle in each of the arithmetic mean waves measured withtwo jigs, and a component wave of a frequency of (½)f. By extendingthis, FIG. 28 illustrates the horizontal axis representing the frequencyand the horizontal axis representing the amplitude value of thearithmetic mean of the arithmetic mean wave having the frequencycomponent. The amplitude is the absolute value of a cosine function.Incidentally, the f in the figure has been calculated for a distance Δlbetween the transducers in accordance with the equations 21 and 22.

FIG. 29 is a cross-sectional view illustrating a concrete material inwhich embedded is a reinforcing bar being detected. A concrete material81 to be used for this detection is 300 mm in length, width, and height.In addition, a total of four deformed reinforcing bars 82, having adiameter of 19 mm, are embedded at positions 75 mm from both sidewalls.Furthermore, two deformed reinforcing bars 82 are embedded to a depth of50 mm from a surface, while the other tow are at 230 mm from thesurface. In addition, thirteen measurement positions P1 to P13 were setin the direction orthogonal to the longitudinal direction of thedeformed reinforcing bars 82.

Then, using two types of jigs at each of the measurement positions, a600-time arithmetic mean was determined (equation 18), respectively, andthen their arithmetic mean y_(ave)(t) was calculated. Furthermore, asthe measuring method at this time, the transmitting transducer and thereceiving transducer were slidably moved as shown in FIG. 11.Incidentally, one jig keeps the transducers spaced apart by 110 mm andthe other jig keeps the transducers spaced apart by 135 mm, with adifference Δl of 25 mm therebetween. The transducers used here are thesame as those used for the measurement of the thickness of the concretematerial according to the aforementioned third embodiment.

FIG. 30 is a schematic view illustrating the transmission of a waveproduced upon detection of the deformed reinforcing bars 82 shown inFIG. 29. The difference between the time of reception of a reflectedwave 83 a from the upper end of the upper reinforcing bar 82 with thetransducers being spaced apart by 110 mm and that with the transducersbeing spaced apart by 135 mm is expressed by the following equation 24in accordance with the equation 21 since the transmission velocity ofultrasonic waves in the concrete material 81 is 4.44 mm/μs.$\begin{matrix}{{\Delta \quad t} = {\frac{2 \times ( {\sqrt{( {13.5/2} )^{2} + 5^{2}} - \sqrt{( {11/2} )^{2} + 5^{2}}} )}{4.44} = {0.44\quad ({µs})}}} & (24)\end{matrix}$

Accordingly, a frequency f_(A) corresponding to this is found to be2,300 kHz from the equation 22. That is, in FIG. 28, these arithmeticaveraged component waves have the maximum amplitude at f=f_(A)=2,300 kHzand the minimum amplitude at (½)f_(A)=1,150 kHz. In addition, as thefrequency fi approaches zero from (½)f_(A), its amplitude increases inaccordance with a cosine function.

On the other hand, the difference Δt in time of reception between waves85 a and 85 b, transmitting on a surface of the concrete material 81,and the frequency f, corresponding to the difference are expressed bythe following equations 25 and 26 for surface waves and by the followingequations 27 and 28 for longitudinal waves, where the velocity of soundis 4.44×0.25 mm/μs. $\begin{matrix}{{\Delta \quad t} = {\frac{25}{( {4.44 \times 0.68} )} = {8.33\quad ({µs})}}} & (25) \\{f_{B} = {\frac{1000}{8.33} \approx {120\quad ({kHz})}}} & (26) \\{{\Delta \quad t} = {\frac{25}{4.44} = {5.63\quad ({µs})}}} & (27) \\{f_{B} = {\frac{1000}{5.63} \approx {180\quad ({kHz})}}} & (28)\end{matrix}$

In addition, the difference Δt in time of reception between direct waves84 a and 84 b and the frequency f_(B) corresponding to the differenceare expressed by the following equations 29 and 30, assuming that theequivalent velocity of the direct wave is about one-half that of thelongitudinal wave. $\begin{matrix}{{\Delta \quad t} = {\frac{25}{( {4.44 \times 0.5} )} = {11.26\quad ({µs})}}} & (29) \\{f_{B} = {\frac{1000}{11.26} = {88\quad ({kHz})}}} & (30)\end{matrix}$

Accordingly, as shown in FIG. 28, these waves have the maximum amplitudewith the frequency fi being equal to f_(B), 2f_(B), 3f_(B) . . . .However, if the component waves of an input ultrasonic wave areattenuated in several waves, the same component wave contained in anarithmetic mean waves is to be attenuated in several waves. For example,suppose that a 50 kHz wave is attenuated in about two waves and a 500kHz or higher wave is attenuated in about one wave. In this case, at afrequency higher than a frequency corresponding to the frequency f_(B),the wave would disappear. FIG. 31 is a view illustrating waves with afrequency component being 2f_(B), and two types of jigs being employed,(a) being a schematic view illustrating arithmetic mean y_(D1)(t)obtained with one transducer being placed at a short distance from theother, (b) being a schematic view illustrating arithmetic mean y_(D2)(t)obtained with one transducer being placed at a long distance from theother, and (c) being a schematic view illustrating their arithmetic meanye(t) obtained in accordance with the equation 20. As shown in FIGS.31(a) and (b), with the wave number of a wave having a cycle of Δt,determined by the equations 21 and 22, being about two waves, theamplitude of their arithmetic mean y_(ave) (t) is one-half that of thearithmetic mean y_(D1)(t) and Y_(D2)(t).

As described above, using the aforementioned two jigs in the measurementshown in FIG. 29, the arithmetic mean y_(ave)(t) is calculated inaccordance with the equation 19 or 20 and then a component wave having agiven center frequency is gained by filtering in the range of frequencyfrom 2f_(B) to (½)f_(A), thereby making it possible to gain a reflectedwave or the like from the reinforcing bar to be detected. FIG. 32 is agraph illustrating the Fourier spectrum of the gained frequencycomponent having a center frequency of 590 kHz, while FIG. 33 is aschematic view illustrating a time series waveform at each position ofmeasurement.

As shown in FIG. 33, a scattered wave, a direct wave, and a longitudinaland surface wave transmitting at a surface of the concrete areeliminated, causing only a reflected wave and the like from the deformedreinforcing bars 82 being detected to emerge at the measurementpositions P3 and P11. That is, it is shown that the deformed reinforcingbars 82 exists generally immediately below the measurement positions P3and P11 and the covering thereof is about 5cm in thickness.Incidentally, the numerical value on the vertical axis representative ofthe thickness of covering in FIG. 33 has been determined from thetransmission velocity of the transverse wave. In addition, one shown inFIG. 33 was obtained by raising an actually obtained waveform to thefourth power.

Furthermore, the following measurement was carried out to determine thethickness of the covering of the aforementioned deformed reinforcingbars 82, having a diameter of 19 mm, with higher accuracy and thediameter thereof. Here, employed were four jigs with the transducers ofone jig being spaced apart by 4 mm longer than those of another. FIG. 34is a schematic view illustrating the distance between the transducers ofeach jig. The jig D1 has a distance l₁ of 40 mm between the transducers,the jig D2 has a distance l₂ of 44 mm between the transducers, the jigD3 has a distance l₃ of 48 mm between the transducers, and the jig D4has a distance l₄ of 52 mm between the transducers. In addition, theiraverage distance l_(ave) is 46 mm. Incidentally, the deformedreinforcing bars 82 have a maximum diameter of 21.5 mm and a minimumdiameter of 18 mm, with the distance between the positions of themaximum diameter being 12 mm.

First, like in the measurement shown in FIG. 11, while the transducersare moved 10 cm on the measurement position P3 of FIG. 29 for each jig,a 1,000-time arithmetic mean y_(Di)(t) was determined. Then, thearithmetic mean yac(t) shown by the following equations 31 wascalculated in accordance with the equation 20. $\begin{matrix}{{y_{ave}(t)} = {\frac{1}{4}{\sum\limits_{i = 1}^{4}{y_{Di}(t)}}}} & (31)\end{matrix}$

Subsequently, a component wave in a bandwidth similar to the Fourierspectrum shown in FIG. 32 at a center frequency of 690 kHz was gained.Then, the amplitude of this wave was raised to the second power. FIG. 35is a graph illustrating time series waveforms obtained for this case. Inaddition, Table 1 below shows the time of generation of each of thegenerated waves in FIG. 35. Incidentally, the time of generation is alsoshown by reference in Table 1 for waves gained with a center frequencyof 1,200 kHz.

TABLE 1 Frequency (kHz) 91 92 93 94 95 96 97  690 23.3 25.8 30.9 35.938.1 41.7 43.7 1200 22.0 25.0 30.1 33.5 37.2 39.3 41.3

Of these generated waves, the peaks 91 to 97 indicate the reflectedwaves from the deformed reinforcing bars 82 immediately below thetransducers and the waves detouring around the deformed reinforcing bars82 or in the concrete material 81 along the periphery of the deformedreinforcing bars 82.

Now, the thickness of covering and the diameter of the reinforcing baris measured from the time of generation shown in Table 1.

For the thickness of the covering of the reinforcing bar, the peak 91indicates the reflected wave from the upper end of the reinforcing bar82 with a transmission length “a” thereof being 51.73 mm from23.3×4.44/2. In addition, according to the results of a number ofmeasurements, letting d be the thickness of the covering of thereinforcing bar, the following equations 32 holds. $\begin{matrix}{d = \sqrt{a^{2} - ( \frac{l_{ave} - c}{2} )^{2}}} & (32)\end{matrix}$

where c is the diameter of the transducer. Then, substituting a=51.73and the like into the equation 32 gives the thickness of the covering bythe following equation 33. $\begin{matrix}{d = {\sqrt{51.73^{2} - ( \frac{46 - 20}{2} )^{2}} \approx {50\quad ({mm})}}} & (33)\end{matrix}$

Since the actual measurement shows 50 mm as mentioned above, it can besaid that an extremely high accuracy is provided.

On the other hand, the generation indicated by the peaks 92 to 97 is awave passing thorough the deformed reinforcing bar 82 immediately underthe transducer. FIG. 36(a) is a schematic view illustrating a generatedwave corresponding to the peak 92, (b) being a schematic viewillustrating a generated wave corresponding to the peak 93, (c) being aschematic view illustrating generated waves corresponding to the peaks94 and 96, and (d) being a schematic view illustrating a generated wavecorresponding to the peaks 95 and 97.

That is, a generated wave 92 a corresponding to the peak 92 is areflected wave from the lateral edge portion of the reinforcing bar 82.On the other hand, a generated wave 93 a corresponding to the peak 93 isa wave that is refracted at the upper end of the reinforcing bar 82,reflected on the lower end, and then further refracted at the upper end.A generated wave 94 a corresponding to the peak 94 is a longitudinalwave detouring around in the deformed reinforcing bar 82 along theperiphery of the deformed reinforcing bar 82, while a generated wave 96a corresponding to the peak 96 is a like transverse wave. On the otherhand, a generated wave 95 a corresponding to the peak 95 is alongitudinal wave detouring around in the deformed reinforcing bar 82along the periphery of the deformed reinforcing bar 82, while agenerated wave 97 a corresponding to the peak 97 is a like transversewave.

The aforementioned judgment was made in accordance with the results ofmeasurements of the same type of about 200 examples in consideration ofreproducibility of measurement.

Theoretically, the order of generation of waves passing through adeformed reinforcing bar can be sorted out as follows in accordance withtheir path length and their velocity of sound. FIG. 66 is a schematicview illustrating the order of generation at various paths.

Referring to FIG. 66, generated waves 101 a and 101 b are longitudinalreflected waves from the upper end of the reinforcing bar, the formerfrom a position of the maximum diameter and the latter from a positionof the minimum diameter.

A generated wave 102 is a longitudinal reflected wave from a projectiondisposed in the longitudinal direction of the deformed reinforcing bar.

Generated waves 103 a and 103 b are longitudinal waves from the path ofthe generated wave 93 a, the former from the position of the minimumdiameter and the latter from the position of the maximum diameter.

Generated waves 104 a and 104 b are longitudinal waves detouring aroundin the reinforcing bar in the path of the generated wave 94 a, theformer from the position of the minimum diameter and the latter from theposition of the maximum diameter.

Generated waves 105 a and 105 b are longitudinal waves of the path ofthe generated wave 95 a, the former from the position of the minimumdiameter and the latter from the position of the maximum diameter.

Generated waves 106 a and 106 b are transverse waves detouring around inthe reinforcing bar in the path of the generated wave 94 a, the formerfrom the position of the minimum diameter and the latter from theposition of the maximum diameter.

Generated waves 107 a and 107 b are transverse waves detouring around inthe concrete material in the path of the generated wave 97 a, the formerfrom the position of the minimum diameter and the latter from theposition of the maximum diameter.

All the aforementioned generated waves transmit in the form oflongitudinal waves in the concrete material. Waves having acomparatively large amount of energy other than those include a wavethat transmits in the form of a longitudinal wave through a go path inthe concrete material and in the form of a transverse wave through thereturn path. Generated waves 109 a, 109 b, 1210 a, and 110 b are such awave.

That is, the generated waves 109 a and 109 b take the path of thegenerated wave 96 a to detour in the form of a transverse wave around inthe reinforcing bar, transmitting in the form of a longitudinal wave ina go path in the concrete material and in the form of a transverse wavein the return path. The former is from the position of the minimumdiameter and the latter is from the position of the maximum diameter.

The generated waves 110 a and 103 b take the path of the generated wave97 a to detour in the form of a transverse wave around in the concretematerial, transmitting in the form of a longitudinal wave in a go pathin the concrete material and in the form of a transverse wave in thereturn path. The former is from the position of the minimum diameter andthe latter is from the position of the maximum diameter.

The generated waves of FIG. 35 obtained as the results of theaforementioned measurement indicate low-frequency component waves havinga center frequency of 690 kHz. At such a level of frequency, for thereflected waves from the deformed reinforcing bar and the detouringwaves, a wave from the position of the minimum diameter is superimposedupon one from the position of the maximum diameter, respectively.

As described above, the generated waves of FIG. 35 were obtained withthe peak 91 as the superimposition of the generated wave 101 a upon thegenerated wave 101 b, the peak 92 as the generated wave 102 itself, thepeak 93 as the superimposition of the generated wave 103 a upon thegenerated wave 103 b, the peak 94 as the superimposition of thegenerated wave 104 a upon the generated wave 104 b, the peak 95 as thesuperimposition of the generated wave 105 a upon the generated wave 105b, the peak 96 as the superimposition of the generated wave 106 a uponthe generated wave 106 b, and the peak 98 as the superimposition of thegenerated wave 108 a upon the generated wave 108 b. In FIG. 35, thegeneration of the superimposed wave of the generated waves 109 a and 109b and the superimposed wave of the generated waves 110 a and 110 b wasnot found; however, the generation was often recognized in othermeasurements of the same type.

Accordingly, various values can be determined for the type of thereinforcing bar as follows. First, from the time of generation of thepeaks 91 and 93, the maximum diameter can be determined as the followingequations 34 and 35.

Δt=30.9−23.3=7.6  (34)

$\begin{matrix}{\Phi = {\frac{7.6 \times 5.9}{2} = {22.4\quad ({mm})}}} & (35)\end{matrix}$

A value of “l” of the maximum position that is obtained from the Snell'slaw is 22.5 mm, which is highly accurate.

In addition, from the time of generation of the peaks 91 and 94, themaximum diameter can be determined as the following equation 32 usingthe equation 17.

Δt=33.5−22.0=11.5  (36)

$\begin{matrix}{\Phi = {{11.5 \times \frac{5.9}{\pi}} = {21.6\quad ({mm})}}} & (37)\end{matrix}$

Furthermore, from the time of generation of the peaks 91 and 95, themaximum diameter can be determined as the following equations 38 and 39.

Δt=37.2−22.0=15.2  (38)

$\begin{matrix}{\Phi = {{15.2 \times \frac{4.44}{\pi}} = {21.5\quad ({mm})}}} & (39)\end{matrix}$

An actual maximum diameter is 21.5 mm, which is highly accurate.

Furthermore, from the time of generation of the peaks 91 and 96, theminimum diameter can be determined as the following equations 40 and 41.

Δt=39.3−22.0=17.6  (40)

$\begin{matrix}{\Phi = {{17.6 \times \frac{3.2}{\pi}} = {18\quad ({mm})}}} & (41)\end{matrix}$

Furthermore, from the time of generation of the peaks 91 and 97, theminimum diameter can be determined as the following equations 42 and 43.

Δt=41.3−22.0=19.3  (42)

$\begin{matrix}{\Phi = {{19.3 \times \frac{4.44 \times 0.68}{\pi}} = {18.5\quad ({mm})}}} & (43)\end{matrix}$

As described above, the thickness of the covering and the shape of thereinforcing bar can be measured with an extremely high accuracy.Therefore, it is possible to determine whether the reinforcing bar is around reinforcing bar or a deformed reinforcing bar. Incidentally, thepresence or absence of the generation of the peak 92 may bealternatively used to determine whether the reinforcing bar is adeformed reinforcing bar or a round reinforcing bar.

Furthermore, for a corroded reinforcing bar in a concrete material,ultrasonic waves do not transmit inside the reinforcing bar, nevercausing the peaks 93, 94, and 96 of the aforementioned measurement to begenerated. Accordingly, it is possible to determine the level ofcorrosion of the reinforcing bar in accordance with the strength of thepeaks. This makes the present invention highly useful from the viewpointof maintenance and protection of the concrete material.

Furthermore, for a member such as a polyvinyl chloride pipe beingembedded in the concrete material, it is also possible to measure thedepth of the buried member and the diameter thereof.

Incidentally, this embodiment of measurement is to determine anarithmetic mean wave using the four jigs in accordance with theequations 19 and 20. In this case, the equation 44 is used instead ofthe equations 21 and 22 for the frequency of the component wave to beamplified by arithmetic averaging. $\begin{matrix}{f \approx \frac{10^{6} \times V}{\frac{1}{2}\Delta \quad L}} & (44)\end{matrix}$

where ΔL is the difference between the maximum and minimum distancesbetween the transducers. Suppose that the ultrasonic wave in theaforementioned concrete material 81 is a longitudinal wave having atransmission velocity of 4.44(mm/μs) and the equivalent sound velocityof a direct wave and the like for transmitting between the transducersis 2.7 to 3.5 (mu/μs). In this case, assuming that ΔL=490−400=90 (mm),the frequency for amplifying the direct wave 35, the surface wave 37 a,a longitudinal wave 37 b and the like can be determined by the followingequations 45 to 48. Incidentally, these waves make it difficult toidentify the reflected waves 34 and 36 being detected and thus shouldessentially be eliminated. On the other hand, the equations 45 and 46indicate those of the longitudinal wave 37b, while the equations 47 and48 indicate those of the surface wave 37 a and the direct wave 35.$\begin{matrix}{{\Delta \quad t} = {\frac{9/2}{4.44} = 1.0}} & (45) \\{f_{B} = {\frac{10^{6}}{1.0} = {1000\quad ({kHz})}}} & (46) \\{{\Delta \quad t} = {\frac{9/2}{ 2.7 \sim 3.5} = { 1.3 \sim 1.7}}} & (47) \\{f_{B} = {\frac{10^{6}}{ 1.3 \sim 1.7} = {{ 590 \sim 770}\quad ({kHz})}}} & (48)\end{matrix}$

The aforementioned waves that should be eliminated are theoreticallyamplified at a frequency f of 590 to 770 kHz. Generated waves of suchwaves, when amplified, cause the reflected waves 34 and 36 beingdetected to be buried and thus made undetectable. However, in theaforementioned example of measurement, a broadband frequency componenthaving a center frequency of 680 kHz was gained and thereby thegeneration of the desired peaks 91 to 97 were allowed to appear in asignificantly distinct manner. Incidentally, though not illustrated, thesame holds for a center frequency of 1,200 kz. This is because of thefollowing reasons.

FIG. 37 is a graph illustrating a time series wave of a broadbandfrequency component gained from input ultrasonic waves with the centerfrequency being at 1100 kHz. As shown in FIG. 37, the wave has generallyone cycle (μs). After having been input into a concrete material, thisultrasonic wave transmits and then received by a receiving transducer orthe like, while being reflected, refracted, and subjected to modeconversion, and attenuated. At this time, the aforementioned one-cycleinput wave is attenuated and diminished in amplitude to yield a 1,100kHz component wave of a received wave.

Accordingly, for such a component wave of only about one cycle, thosefrequency component waves having a frequency equal to or higher than thefrequency determined by ΔL are not amplified even when the arithmeticaveraging is performed in accordance with the equation 19 or 20.

FIGS. 38(a) through (d) are schematic views illustrating waves obtainedwhen each jig is used and 38(e) is a schematic view illustrating theirarithmetic mean. In the aforementioned measurement, as shown in FIG.38(e), the direct wave or the like included in the arithmetic meany_(ave)(t) resulting from the arithmetic averaging that is performed onfour waves in accordance with the equation 20 is one-fourth the originalwave in amplitude. Furthermore, referring to FIG. 35, the result israised to the second power to be displayed, thereby making its apparentamplitude (¼)²={fraction (1/16)}.

On the other hand, concerning the reflected wave or the like from thereinforcing bar being detected, when a distance of 40 mm between thetransducers is varied three times each by ΔL=3 mm and-thereby becomes 49mm, the amount of variation ΔL, is determined by the following equation49. $\begin{matrix}{{\Delta \quad L_{t}} = {{\sqrt{50^{2} + ( \frac{40 + 9}{2} )^{2}} - \sqrt{50^{2} + ( \frac{40}{2} )^{2}}} = {1.83\quad ({mm})}}} & (49)\end{matrix}$

Accordingly, the frequency corresponding to this is expressed by thefollowing equations 50 and 51. $\begin{matrix}{{\Delta \quad t} = {\frac{1.83/2}{4.44} = 0.20}} & (50)\end{matrix}$

 f _(A)=5.0 (MHz)  (51)

Thus, the reflected wave from the reinforcing bar being detected willhave the minimum amplitude at about 2.5 MHz or one-half of 5.0 MHz, andsubsequently will be amplified as the wave is shifted toward lowerfrequencies.

From this, it is possible to recognize the generation of waves such asreflected waves and detouring waves, which pass through the reinforcingbar being detected, with considerably high accuracy if broadbandcomponent waves are gained at a given center frequency within the rangeof αf_(B) to (½)f_(A).

By the aforementioned third embodiment, a plurality of jigs areemployed. In that case, the surface and direct waves that interfere withdetection would be eliminated most efficiently in a low-frequency regionby gaining a component wave from the arithmetic mean wave at a centerfrequency of one-half of the value of “f” of the equation 44. The reasonfor this is described below.

Consider the (k+1) arithmetic mean waves that use D₁, D₂ , . . .D_(k+1), shown in FIG. 21. It is assumed that the component waves havinga given frequency of these arithmetic mean waves have substantially thesame strength. FIG. 39 is a view illustrating the procedure ofarithmetic averaging according to the equation 20. The lowermost stage(a₁) shows the measurement position of the (k+12) arithmetic mean waves.Instead of determining the arithmetic mean of these arithmetic meanwaves in accordance with the equation 20, arithmetic averaging isperformed on the adjacent arithmetic mean waves, thereby determining(k+1)/2 arithmetic mean waves as in (a₂). Repeating this processing willprovide (a₁)→(a₂)→(a₃) . . . →(a₆)in sequence.

The wave obtained at the last stage (a₆) is the arithmetic mean waveshown in the equation 20. Assuming that each of the component waves ofthe arithmetic mean waves at the stage (a₁) has the same amplitude, FIG.40 shows how the spectrum of the arithmetic mean waves at each stagechanges. The figure is expressed with the spectrum value at the stage(a₁) being taken as 1.0 over the entire frequency band. The spectrum a₁is the arithmetic mean wave at the stage (a₂) and shows the frequencyregion of 0 to ({fraction (2/4)})f in the spectrum of FIG. 28. Thefrequency (½)f is the frequency at which the surface and direct wavesthat interfere with detection at the stage (a₂) are eliminated mostefficiently from the arithmetic mean waves in the low-frequency region.

Now, the arithmetic mean wave spectrum at the next stage (a₃) is as ina₂. Similarly, the arithmetic mean wave spectrum at the stage (a₄) is asin a₃. Finally, the arithmetic mean wave spectrum at the stage (a₆) isobtained as shown by a bold line in a₅.

The f₀ value from the (½)f₀=f/32 shown in the figure corresponds to thef value shown by the equation 44. For this reason, the followingequation 52 is defined in place of the equation 44. $\begin{matrix}{f_{0} = \frac{10^{6} \times V}{\frac{1}{2}\Delta \quad L}} & (52)\end{matrix}$

The numerical value of 32 in the foregoing comes from the assumptionthat jigs are 32 in number. Accordingly, jigs that are 128 in numberwould cause the aforementioned numerical value of 32 to be changed to128.

The aforementioned explanation was made for a measurement using a numberof (32) jigs with the D_(i) and D_(i+1) jigs being different by Δl inlength.

It is possible to obtain perfectly the same effect as in theaforementioned measurement without using such jigs by carrying out thescanning of the transducers shown in FIGS. 3 and 6 in a manner such thatthe distance between the transducers has he minimum value of l₁ ad themaximum value of l_(M), and is changed at a constant velocity.

In this case, ΔL to be applied to the equation 52 is given by equation53.

ΔL=l _(M) −l ₁  (53)

On the other hand, in the aforementioned processing of arithmeticaveraging, consider the spectrum of reflected waves of a detected targetsuch as a plate thickness.

This is explained using the measurement diagram of FIG. 24. Referring tothe figure, letting d be the plate thickness, the maximum path length of201 is {(l_(M)/2)²+d²}^(½) and the minimum path length is{(l₁/2)²+d²}^(½). Letting ΔL_(B) be the difference between the pathlengths, the lowest frequency at which the component wave of the platethickness reflected wave disappears is expressed by the followingequation 52 in accordance with the equation 52 in the same manner as inthe foregoing. $\begin{matrix}{{\frac{1}{2}f_{B}} = \frac{10^{6} \times V}{\frac{1}{2}\Delta \quad L_{B}}} & (54)\end{matrix}$

Accordingly, it is possible to obtain the spectrum of the platethickness reflected waves with the vale of “If” of FIG. 40 beingreplaced by f_(B). Letting l=l_(M)−l₁, if the relationship holds thatd>>21 between “l” and “d”, the value of f_(B) is extremely greater thanthe value of “f”. A spectrum corresponding to the a₅ spectrum of FIG. 40is determined using the value of f_(B) and taken as _(B)a₅, and FIG. 41shows the _(B)a₅ and a₅together. This _(B)a₅ is a normalized spectrum ofthe 201 path wave of the arithmetic mean wave that is finally obtained.At the frequency band shown by “⇄” of FIG. 41, the spectrum _(B)a₅ ofthe 201 path wave being detected is significantly prominent relative tothe 202 and 203 surface waves and the 204 direct wave, which interferewith detection. By gaining a time series wave in this frequency band,this makes it possible to obtain a wave, in which only the 201 path waveis extremely prominent, with no mistake.

A measurement which is carried out using the aforementioned equations 52and 53 is shown as a fourth embodiment. In the second embodiment,arithmetic averaging was performed 10,000 times in accordance with thetransducer scanning method shown in FIG. 6, and the arithmetic mean wavewas obtained as shown in FIG. 7. The measurement to be shown correspondsto this measurement. The measurement is carried out by the scanning ofFIG. 6, with L=15 cm, the maximum distance being 15 cm and the minimumdistance being 10 cm between the transducers, and the velocity beingmade constant for varying the distance. FIG. 42 shows the arithmeticmean wave that is obtained with the transmitting and receivingtransducers having an oscillator 40 mm in diameter whose resonantfrequency of 500 kHz.

FIG. 43 illustrates a component wave gained by applying the equations 52and 53 to the arithmetic mean wave of FIG. 42 with the frequency shownin the following equation 55 being employed as the center frequency.$\begin{matrix}{{\frac{1}{2}f} = \frac{\frac{1}{2} \times 10^{6}}{\frac{1}{2}\Delta \quad {L/\overset{\sim}{V}}}} & (55)\end{matrix}$

Here, the equation 55 is rewritten as the following equation 56, basedon the longitudinal-wave sound velocity in-the concrete being 4,300 m/sand assuming that the surface and direct waves, which interferedetection, vary in its equivalent sound velocity within the range of3,000 to 4,000 m/s and thus have an average velocity of 3,500 m/s.$\begin{matrix}{{\frac{1}{2}f} = {\frac{\frac{1}{2} \times 10^{6}}{\frac{1}{2}{( {150 - 100} )/3.5}} = {70\quad ({kHz})}}} & (56)\end{matrix}$

When compared with those in FIG. 42, most of the surface, direct, andscattered waves, which interfere with detection, are eliminated in FIG.43, making it possible to clearly recognize the reflected wave of A′-B′path. Incidentally, the arithmetic averaging was performed 3,000 timesin this measurement.

On the other hand, FIG. 44 shows the method that is considered as amethod for scanning transducers without measuring jigs. FIG. 44(a)illustrates the method shown in the fourth embodiment. FIG. 44(d)illustrates a method for scanning two transducers at a constant velocityin either straight-line or curved-line scanning manner, at random withinthe movement region 7 of FIG. 3 and 11a and 11 b of FIG. 6, from thepoints C to D as shown in FIG. 11 along immediately above the subjectbeing detected, with the distance between the transducers being fixed.On the other hand, FIG. 44(b) shows two transducers brought into contactwith each other in the scanning method shown in FIG. 44(d). FIG. 44(c)illustrates a measurement with one transducer, showing a method forscanning one transducer, serving as the transmit receiving transducers,at a constant velocity (in either straight-line or curved-line scanningmanner), at random within the circular region 7 of FIG. 3 (alternativelywithin an elliptical or a rectangular region), from the points C to D asshown in FIG. 11 along immediately above the subject being detected.

For each of the aforementioned scanning methods, it is shown below howthe equation 52 looks like. It is first explained that a limit value ofa predetermined amount exists commonly with respect to the scanningmethods shown in FIGS. 44(a) to (d) in the value of “f₀” of the equation52 indicating the frequency at which the surface and direct waves, whichinterfere with detection, are eliminated most efficiently in alow-frequency region. A general shape of a longitudinal-wave transduceris shown in FIG. 45. With such a transducer being employed as areceiving transducer, the received wave is mixed with a component waveexcited at the resonant frequency of f_(p) of an outer sheath. Atransducer 1001 is provided with an oscillator 1001 a, a protectivematerial 1001 b for protecting the contact face of a detected material,an attenuating material 1001 c, and an outer sheath 1000 d. Withone-half of the value of f₀ defined by the equation 52 being consistentwith this f_(p) and a component wave being gained at a center frequencyof (½)f₀, the component wave is superimposed upon an excited wave at theresonant frequency of the outer sheath 1001 d. This makes it necessaryto avoid gaining a component wave at such a frequency. According to anumber of experimental measurements, it has been found that the value off₀ should be given by the following equation 57 to avoid this.Incidentally, the letter “S” shown in the figure designates thethickness of the outer sheath.

f ₀=4f _(P)  (57)

Now, it is adjusted how the right-hand side ΔL of the aforementionedequation 52 is expressed for each scanning method.

For the scanning method shown in FIG. 44(a) because of the reasondescribed in the fourth embodiment, the ΔL is determined in accordancewith the equation 53 and then applied to the equation transducer 52 todetermine f₀. Incidentally, with f₀<4f_(P), it may be employed thatf₀=4f_(P). In other words, ΔL my be changed to ΔL=10 ⁶V/(2f_(p)) todetermine f₀.

For the scanning method shown in FIG. 44(d), ΔL is applied to theequation 52 to determine f₀, letting Φ₂ be the diameter of theoscillator inside the receiving transducer as shown in the followingequation 58, assuming that the receiving transducer consists of a set ofsmall transducers and because of the reasons described in the fourthembodiment. However, expressing that f₀=4f_(P) at f₀<4f_(P) and changingΔL to ΔL=10⁶V/(2f_(p)) may be allowed to determine f₀.

ΔL=Φ₂  (58)

Incidentally, in the method in which an arithmetic mean wave isdetermined for each jig for holding the distance between the twotransducers shown in the third embodiment, using the scanning methodshown in FIG. 44(d), to determine the arithmetic mean of these twoarithmetic mean waves, ΔL may be given as in the following equation 59using the difference Δl in length between the two jigs. This ΔL isapplied to the equation 52 to determine ₀. However, expressing thatf₀=4f_(P) at f₀<4f_(P) and changing ΔL to ΔL=10⁶V/(2f_(p)) may beallowed to determine f₀.

ΔL=2×l  (59)

For the scanning method shown in FIG. 44(b), as shown in the followingequation 60, the ΔL used in the equation 52 may be employed as thepredetermined amount G₁, which is defined by the material of the bodybeing detected, the diameter of the oscillator in the transducers, thethickness of the outer sheath of the receiving transducer, and theresonant frequency of the outer sheath.

ΔL=G ₁  (60)

For the scanning method shown in FIG. 44(c), as shown in the followingequation 61, the ΔL used in the equation 52 may be employed as thepredetermined amount G₂, which is defined by the material of the bodybeing detected, the diameter of the oscillator in the transducers, thethickness of the outer sheath of the receiving transducer, and theresonant frequency of the outer sheath.

ΔL=G ₂  (61)

Shown below is the amount given to the predetermined amount G₁ in theaforementioned scanning method shown in FIG. 44(b).

FIG. 46 is a view illustrating a longitudinal wave 404 a, a transversewave 404 b, and a surface wave 404 c, transmitting on a concrete surfaceto a receiving transducer 402, with a longitudinal ultrasonic wave 403being input to the concrete surface directly downwards from atransmitting transducer 401. Here, the figure is drawn, letting thethickness S of the outer sheath of the transducers be zero.

Reference numeral 407 schematically indicates the strength of thecombined wave of the waves 404 produced for each transmit ultrasonicwave of 403, which is discretely indicated. Reference numeral 408 is anenvelope of these strengths, illustrating the aforementioned combineswave being suddenly attenuated. Following the introduction of theequation 52 showing a value two times greater than the frequency atwhich the component such as the surface and direct waves, whichinterfere with detection, is reduced to a minimum in the low frequencyregion, such an assumption has to hold for the receiving transducer ofFIG. 46 that the strength of each of the frequency component waves,received at the discrete reception points S₀, S₁, S₂, . . . S_(n), isgenerally equal to each other. To satisfy the condition, a dotted line409 was set, the reception points were divided into two regions 405 and406, and then it was assumed in the figure that the strength of receivedcomponent waves at the S₀ to S₄ was equal to each other and the strengthof received component waves at the S₅ to S_(n), was equal to each other.Here, the distance between the aforementioned dotted line 409 and acenter line 401 of the transmitting transducer was defined as β.

Then, the strength of the component wave of the region 405 is comparedwith that of the region 406 to neglect the latter since the former isextremely larger than the latter. Assuming that the reception points S₀,S₁. . . S₄ are each a receiving transducer of a small diameter and dueto the reason shown in the fourth embodiment, the predetermined amountG₁ of the equation 60 for determining the frequency (½) f₀ of thelowest-frequency component wave, reduced in strength, of the waves 404a, 404 b, 404 c, which interfere with detection, can be introduced as inthe following equations 62 and 63. $\begin{matrix}{G_{1} = {\beta - \frac{\Phi_{1} + {2S}}{2}}} & (62)\end{matrix}$

where Φ₁ is the diameter of the oscillator in the transmittingtransducer, Φ₂ is the diameter of the oscillator in the receivingtransducer, S is the thickness of the outer sheath of the aforementionedtransducers, and f_(p) is the resonant frequency of the outer sheath ofthe receiving transducer.

Here, it holds that G₁=Φ₂ when (Φ₂<G₁.

With the value of f₀ obtained by applying the aforementioned G₁ to theequations 60 and 52, it holds that f₀=4f_(P) when f₀<4f_(P). In otherwords, the value of G₁ is given by the equation 63. $\begin{matrix}{G_{1} = \frac{10^{6} \times V}{2 \times f_{P}}} & (63)\end{matrix}$

Incidentally, it has been found from a number of experimentalmeasurements that the value of β takes on the following numerical valuesfor typical concrete having a strength of 350 to 450 kg/cm².

An oscillator of diameter 40 mm in the transmitting transducer: β=50˜53mm

An oscillator of diameter 76 mm in the transmitting transducer: β=65˜68mm

Furthermore, it is shown below what value is given to the predeterminedamount G₂ in the scanning method shown in FIG. 44(c). Assuming that thetransmitting and receiving transducers are a set of discrete transducersof a small diameter, the predetermined amount G₂ can be introduced as inthe equation 64, following the introduction of the aforementionedpredetermined amount G₁ and the introduction of the equations 52 and 53.That is,

G ₂=Φ  (64)

where Φ is the diameter of the oscillator of the transmitting andreceiving transducers.

Moreover, letting f_(p) be the resonant frequency of the outer sheath ofthe transducer, the value of G2 is given by the equation 65 when itholds for the value of 0 calculated by the equations 52, 61 and 64thatf₀<4_(P). $\begin{matrix}{G_{2} = \frac{10^{6} \times V}{2 \times f_{P}}} & (65)\end{matrix}$

Incidentally, the value of f₀ obtained by the equation 52 increases whenmeasurements are made using the scanning method shown in FIG. 44(b) or(c). According to the scanning method shown in FIG. 44(b) in which atransducer having an oscillator 40 mm in diameter, a frequency of 500kHz, and an outer sheath of thickness 10 mm, the equation 66 holds inaccordance with the equation 62. $\begin{matrix}{G_{1} = {{50 - \frac{40 + {2 \times 10}}{2}} = 20}} & (66)\end{matrix}$

Then, suppose that the average sound velocity of waves, which interferewith detection, is 3,500 to 4,000 m/s in the equation 52 and 60. In thiscase, the f₀ shown in the following equation 67 is obtained.$\begin{matrix}{f_{0} = {\frac{10^{6} \times 3.75}{\frac{1}{2} \times 20} = {380\quad ({kHz})}}} & (67)\end{matrix}$

Consequently, the frequency at which the lowest-frequency waves, whichinterfere with detection, are reduced to a minimum will be at aboutf₀=190 kHz.

It is not rare that the ultrasonic waves in the concrete areacceleratingly attenuated due to the scattering phenomenon as thetransmission distance increases, making the attenuation proportional tothe second to third power of the transmission distance. In themeasurement of plate thickness of 50 cm, 100 cm, or more, it isdifficult in some cases to measure the aforementioned waves of afrequency of about 190 kHz due to attenuation caused by scattering. Asan example, FIG. 47 illustrates an embodiment of a measurement made atpoint Δl of the concrete model of FIG. 72 according to the scanningmethod shown in FIG. 44(b), using the transmitting and receivingtransducers having an oscillator 40 mm in diameter and a frequency of500 kHz. The presence of reflected waves cannot be recognized at theposition, indicated by the arrow, where a plate thickness reflected waveis theoretically generated. This is because the remaining surface waves,which interfere with detection, have a relatively larger amplitude thanthat of the plate thickness reflected wave that is significantlyattenuated due to its long transmission distance.

FIG. 48 illustrates the comparison between the spectra of interferencewaves, which interfere with detection, included in such a received waveand of waves of a target to be detected such as a plate thickness.

Designated as 501 is an interfering wave, which interferes withdetection, included in an arithmetic mean wave, while designated as 601is the spectrum of a detection target wave. The interfering wave wouldbe significantly attenuated through a long transmission distance likethe detection target wave. Therefore, the time series wave of thespectrum 501 of the interfering wave must be concentrated at an earliertime of generation in the arithmetic mean wave. From this, a time seriesfilter is defined as in the equation 68. $\begin{matrix}{{G(t)} = {\sin ( {\frac{\pi}{2} \cdot \frac{t}{t_{0}}} )}} & (68)\end{matrix}$

where t₀ is defined by the equation 69, letting {tilde over (d)}be thedepth of a target being detected. $\begin{matrix}{t_{0} = \frac{2\overset{\sim}{d}}{V}} & (69)\end{matrix}$

Multiplying the arithmetic mean wave by to would cause 501 to be reducedto a spectrum 502, and further multiplications would cause 502 to bereduced to 503 and 503 to 504, thus reducing the spectrum values asshown in the figure. At a stage where the spectrum of the interferingwave included in the arithmetic mean wave has been reduced to 504, thedetection target wave spectrum 601 is extremely greater than theinterfering wave 504 in the range of frequencies of (½)f₀ to f_(D).Thus, by shifting the center frequency for gaining a component wave fromthe arithmetic mean wave from (½)f₀ to as low a frequency as possible,the attenuation due to transmission scattering is reduced as thecomponent wave has a lower frequency, thereby possibly causing thedetection target wave to emerge. Therefore, f_(D) is given by theequation 70 using the resonant frequency f_(p) of the outer sheath ofthe aforementioned receiving transducer.

f _(D)=4f _(P)  (70)

As described above, to obtain a reflected wave from a deep positionusing the scanning method shown in FIG. 44(b) or (c), the value of to,which is determined from a predetermined detected depth {tilde over(d)}, is determined in accordance with the equation 69. The to isapplied to the equation 68 and then the aforementioned arithmetic meanwave is multiplied by the resulting time series filter a plurality oftimes. Then, a center frequency is set within the range of the value of(½)f₀, determined by the combination of the equations 60 and 52 or thecombination of equations 61 and 52, to f_(D) of the equation 70. Then, acomponent wave is gained while the setting is gradually being shiftedfrom (½)f₀ to f_(D).

The acquisition of a reflected wave from a deep position using theaforementioned scanning method shown in FIG. 44(b) or (c) is shown as afifth embodiment. Nevertheless, the scanning methods shown in FIGS.44(b) and (c) are essentially the same. The fifth embodiment is shown inaccordance with the scanning method shown in FIG. 44(b). An explanationis made using an embodiment of measurement of FIG. 42 of the platethickness reflection with a plate thickness being 50 cm, which employsthe concrete model of FIG. 72. FIG. 49 illustrates a wave of FIG. 42which has a center frequency of 200 kHz and is multiplied three times bya time series filter function G or G(t)=sin ((π/2) (t/400) with a depthto be detected being given about 80 cm, or t₀ being given 400 μs in FIG.69, which is applied to the equation 68. AT this point in time,something like a plate thickness reflection can be seen at the positionindicted by the cursor; however, it is difficult to identify this as aplate thickness reflection. Thus, such an analysis is carried out inwhich the center frequency for gaining a component wave is graduallyshifted from (½)f_(0 to f) _(D).

As described above, (½)f₀ is 190 kHz. The value of f_(D) is found to be4×16.5≈65 kHz in accordance with the equation 70 in which thetransducers used have a resonant frequency of about 16.5 kHz. Omittingthe intermediate course, FIG. 50 illustrates the result of a gainedcomponent wave with the center frequency being at f_(D)=65 kHZ. It ispossible to recognize, astonishingly in a distinct manner, the platethickness reflected wave that could not be recognized in the componentwave near 200 kH (FIG. 47).

An embodiment of measurement is shown as a sixth embodiment in which theaforementioned component wave gained with center at one-half of f₀ isshifted towards high frequencies or low frequencies.

FIG. 51 illustrates a model of concrete used for measurement. The modelhas a plane of 30 cm×30 cm and a thickness of 35 cm, with a roundreinforcing bar 900 of diameter 19 cm being embedded at a position of 10cm in covering thickness from a surface. At measurement points 1 to 5,using the scanning method shown in FIG. 44(b), transducers are scannedacross a movement width of 20 cm in parallel to the longitudinaldirection of the embedded reinforcing bar. Arithmetic averaging wasperformed 3,000 times at each point, and both the transmitting andreceiving transducers used have an oscillator of diameter 40 mm and aresonant frequency of 1 MHz with each transducer being 60 mm indiameter.

The value of f₀ in this measurement is the same as that of the fifthembodiment. That is, (½)f₀=190 kHz.

FIG. 52 illustrates component waves gained at each measurement pointwith the center frequency being at 190 kHz. At this frequency, aninterfering wave has disappeared and only a reflected wave from thereinforcing bar disposed immediately under the measurement point 3 canbe distinctly recognized. On the other hand, FIG. 53 illustrates anexample obtained in the course of shifting the center frequency forgaining a component wave from the wave that has been obtained bymultiplying ten times each component wave of FIG. 52 by the time seriesfilter, G(t)=sin(πt/(2×230), obtained by applying t₀=(2×500/4.3≈230 μsto the equation 68, the t₀ being given by the equation 69 with the depthto be detected being made equal to 50 cm. Here, the center frequency isshifted, by filtering, from the aforementioned (½)f₀ (=190 kHz) to thevalue (60 kHz) obtained by successively applying the resonant frequencyof the outer sheath of the receiving transducer (16.5 kHz for thetransducers used) to the equation 70. The example has a gaining centerfrequency of 150 kHz. Both of the reflected waves from the reinforcingbar and indicative of the plate thickness are allowed to emergedistinctly. This is an example which allows for gaining a reflected waveindicative of the thickness even at frequencies higher than 4f_(D)=60kHz since the thickness is relatively as thin as 35 cm. Incidentally,FIGS. 52 and 53 illustrate a gained component wave that is raised to thefourth power.

By the aforementioned analysis, or in FIGS. 52 and 53, it is possible tomeasure the planar position of presence and the thickness of thereinforcing bar; then, how the diameter of the reinforcing bar can bemeasured?

FIG. 52 shows the component wave gained at 200 kHz and FIG. 72 at 150kHz. It is impossible to acquire reflected waves for recognizing thediameter of the reinforcing bar with component waves of such lowfrequencies. In this regard, it is necessary to shift the centerfrequency for gaining a component wave towards higher frequencies. Amethod employed for this purpose is described.

FIG. 54 illustrates a 200 kHz component wave provided by measurement 3.Since high-frequency components of a reflected wave from the targetbeing detected are reduced in strength as the depth of the subject beingdetected becomes comparatively deeper, gaining a high-frequencycomponent will allow an interfering wave, which has disappeared, torelatively emerge.

In this regard, prior to sweeping in higher frequencies, the position ofthe reflected wave from the reinforcing bar of FIG. 54, indicated by thecursor, is designated as to, which is applied to the equation 68 toprepare the time series function G(t), by which the component wave of200 kHz of FIG. 54 is in turn multiplied a plurality of times. In thisembodiment, the multiplication is carried out three times. Ahigh-frequency component wave is gained from a wave on which such timeseries filtering has been performed. FIG. 55 illustrates an amplifiedcomponent wave obtained at a center frequency of 680 kHz reached aftergradual sweeping of center frequencies towards higher frequencies. Awave 801, having a large amplitude, on the leftmost is a reflected wavefrom the upper end of the reinforcing bar. The position indicated by thecursor is its time of generation.

The cursor to the right indicates the position of generation of areflected wave that transmits through a go path 701 in the form of alongitudinal wave and through a return path 702 in the form of atransverse wave. In addition, the cursor further to the right indicatesthe position of generation of a reflected wave that allows a traceamount of transverse wave transmitted from the transducer to transmitthrough the paths 701 and 702 in the form of a transverse wave. Itshould be noted that a reflected wave 805 with a trace amplitude isgenerated at this position.

Designated as 802 is a reflected wave 76, from the lower end of thereinforcing bar shown in FIG. 19(c), which transmits through the go andreturn paths within the concrete in the form of a longitudinal wave andthrough the reinforcing bar also in the form of a longitudinal wave.

Designated as 803 is a wave which transmits through the reinforcing baron the circumference of the reinforcing bar of FIG. 20(a) in the form ofa longitudinal wave and through the go path 701 and the return path 702within the concrete also in the form of a longitudinal wave.

Designated as 804 is a two-wave-superimposed wave which transmitsthrough the reinforcing bar of FIG. 20(a) on the circumference thereofin the form of a transverse wave and within the concrete in the form ofa transverse wave, and which transmits through the go and return paths701 and 702 and within the concrete in the form of a longitudinal wave.Furthermore, designated as 806 is the generation of a very special wave.The longitudinal wave input into the concrete from the transmittingtransducer is subjected to a mode conversion at the interface betweenthe concrete and a number of fine stones and gaps within the concrete.The superimposed transverse wave produced through this conversion isreflected on the upper end of the reinforcing bar and transmits throughboth the paths 701 and 702 in the form of a transverse wave.

From the foregoing, letting t₂ be the time of generation of the wave801, t₁ be the time of generation of the waves 802, 803, 804, 805, andV_(P) be the longitudinal-wave sound velocity of the ultrasonic waves inthe iron material, the diameter d of the reinforcing bar is calculatedin accordance with the following equation 71 using the waves 801 and802. $\begin{matrix}{d = \frac{( {t_{1} - t_{2}} ) \times V_{P}}{2}} & (71)\end{matrix}$

The diameter d of the reinforcing bar is calculated in accordance withthe equation 17 using the waves 801 and 803.

The diameter d of the reinforcing bar is calculated in accordance withthe following equation 72 using the waves 801 and 804. $\begin{matrix}{d = \frac{( {t_{1} - t_{2}} ) \times V_{S}}{\pi}} & (72)\end{matrix}$

where V_(S)=0.53V_(P).

Incidentally, suppose the wave 804 can be separated into two waves bygaininig a high-frequency component wave. In this case, of these twowaves, letting t₁ be the time of generation of the wave that is producedearlier in time, the diameter of the reinforcing bar can be calculatedin accordance with the aforementioned equation 72, and letting t₁ be thetime of generation of the wave that is produced later in time, thediameter of the reinforcing bar can be calculated in accordance with thefollowing equation 73. $\begin{matrix}{d = \frac{( {t_{1} - t_{2}} ) \times {{}_{}^{}{}_{}^{}}}{\pi}} & (73)\end{matrix}$

where _(c)V_(s)is the velocity of a transverse wave of the reflectedwave within the concrete and determined to be 0.59 to 0.62 of thelongitudinal wave _(c)V_(p). From the foregoing, 801 and 802 give thediameter of the reinforcing bar d=(50.9−44.5)5.9/2=18.9 mm, 801 and 803give d=(54.5−44.5)5.9/π=18.8 mm, and 801 and 804 gived=(65−44.5)5.9×0.53/π=20.4 mm, all of which allow the actual value of 19mm to be measured with extremely high accuracy.

Furthermore, sweeping in higher frequencies possibly causes the waves802, 805, 806 to be diminished in amplitude. FIG. 56 illustrates acomponent wave having a center frequency of 1 MHz.

Incidentally, FIGS. 55 and 56 illustrate a component wave raised to thethird power.

In the aforementioned embodiment, a component wave having apredetermined center frequency was gained from a received wave byfiltering the aforementioned received wave. Though not illustrated, inan ultrasonic detection apparatus with a mechanism having a transmittingtransducer for outputting an oscillating ultrasonic wave of theaforementioned predetermined center frequency and a receiving transducerfor measuring a received wave, the received ultrasonic wave is generallythe same as the component wave gained by filtering in the aforementionedembodiment. By using such an ultrasonic detection apparatus having theaforementioned mechanism, it is possible to obtain, as a received wave,a component wave that is equivalent to the component wave of theaforementioned predetermined center frequency.

According to the aforementioned measuring method, it is possible tomeasure the planar position, the thickness of the covering, and thediameter of the reinforcing bar with high accuracy; however, a concretematerial that has been subjected to weather damage and aging has adeterioration in physical property of the surface layer and numerouscracks of fine widths even when the surface of the concrete materiallooks comparatively good. FIG. 57 is a schematic view illustrating thetransmission of various waves in a concrete material that has beensubjected to aging.

In an attempt to measure the covering thickness and the like of such aconcrete material, the transmission of reflected waves 111 and the likefrom a reinforcing bar 112 being detected is blocked by cracks 115 of afine width. On the other hand, a larger number of direct waves 113,which transmit through deep paths, are received at reception point A2.Accordingly, by the aforementioned method, it is in some cases possibleto detect the planar position of the reinforcing bar but impossible insome other cases. Incidentally, the path of a surface wave 114 to thereception point A2 is also blocked by the crack 115.

Even in such a case, use of the transmission path of an ultrasonic waveof a critical refracted wave that transmits on the surface of thereinforcing bar makes it possible to positively detect the position ofthe reinforcing bar with an extremely high accuracy. FIG. 58 is aschematic view illustrating the path of critical refracted waves.

As described above, suppose numerous cracks 125 have been produced onthe surface. In this case, since the transmission of a reflected waveand the like is blocked by the cracks 125, a wave 121 transmitting as acritical refracted wave via a reinforcing bar 122 and a direct wave 123through a deep layer of the concrete material transmit as the ultrasonicwaves input at transmission point A1 and received at reception point A2.

Here, for the paths for the wave 121 transmitting as a refracted waveand the direct wave 123, the former is shorter than the latter. Inaddition, the transmission velocity of ultrasonic waves is greater inthe reinforcing bar 122 than in the concrete material. Accordingly, atreception point A2, the wave 121 transmitting as a refracted wave isreceived earlier than the direct wave 123. In addition, the greater thedistance between the transmission point Δland the reception point A2,the larger the difference between their reception times becomes.

Incidentally, since the wave 121 transmitting as a refracted wave has anextremely small amplitude, the presence of an extremely low level ofelectrical noise or a disturbance in measurement environments wouldcause the wave 121 transmitting as a refracted wave to be buriedtherein, thereby conventionally making it difficult to detect the wave121.

In this regard, as the results of intense study made by the inverter ofthe present invention, it was found that measurements could be made withan extremely high accuracy on a concrete material having cracks formedon the surface thereof. This was accomplished by incorporating anultrasonic transmitting circuit (the stepped-voltage generator circuit 1a and the stepped-voltage driving circuit 1 c) and a receiving circuit(the amplifier circuit 4 a) into the transducers, respectively, toelectrically separate the circuits from each other, thereby reducingstanding or non-standing electrical noise as much as possible. Then,provided was an apparatus for performing arithmetic averaging atextremely high speeds on a received wave to eliminate still remainingnon-standing electrical noise and disturbances of high energy and thenmeasurements were made in accordance with the method shown below.

FIG. 59 is a schematic view illustrating a method for detecting areinforcing bar in a concrete material on the surface of which cracksare formed. First, with the transmitting transducer and the receivingtransducer being spaced apart from each other by L, a measurement ismade between a transmission point Δland a reception point A2. Δt thistime, measurements are repeated 1,000 times to 2,000 times to performarithmetic averaging or the arithmetic averaging is performed 10,000times or 20,000 times in some cases. Δit this time, it is not necessaryto move the transmitting transducer and the receiving transducer asshown in FIG. 30 to perform the arithmetic averaging of the equation 19or 20. This is because waves such as the surface wave 114, whichinterfere with detection, and the direct wave 113 through a shallow pathare blocked by the cracks 115. Thereafter, in accordance with the samemethod, measurements are made between a transmission point B1 and areception point B2 as well as between a transmission point C1 and areception point C2. Incidentally, L is the distance between thetransmission point B1 and the reception point B2, and between thetransmission point C1 and the reception point C2.

According to this method, like a measurement between the transmissionpoint B1 and the reception point B2, a wave of a critical refracted wavemay be received without causing the cracks to block the transmission ofthe waves through paths 131 and 133. Incidentally, the transmission ofwaves through a path 132 is not blocked irrespective of the presence ofa crack deeper in depth than the embedded reinforcing bar. In addition,the transducers are moved in the direction of arrangement of thereinforcing bar being detected.

Now, the results obtained by an actual measurement in accordance withthe aforementioned method are explained below. FIG. 60 is a viewillustrating a concrete material that has been left for five years driedafter poured, (a) being a plan view thereof, (b) being a cross-sectionalview taken along line D—D of (a) and (c) being a cross-sectional viewtaken along E—E of (a 0. The dimensions of a concrete material 141 inthe vertical and horizontal are each 50 cm, with a thickness of 30 cm.Furthermore, a total of six round reinforcing bars 142, each having adiameter of 19 mm, are embedded at a position of 5 cm from the front andreverse surfaces. Now, at each of measurement positions P21 to P35,letting L be 30 cm, obtained was a received wave after 1,000 times ofarithmetic averaging from a measurement with the distance between thetransmitting transducer and the receiving transducer being fixed.

FIG. 61 is a view illustrating waves received at the measurementposition P28, (a) being a graph illustrating a case where standing ornon-standing electrical noise and disturbance has never been eliminatedand (b) being a graph illustrating a case where they have beeneliminated. That is, FIG. 61(b) illustrates a wave from which electricalnoise and disturbance have been eliminated using the aforementionedarithmetic averaging in accordance with the equation 1. Incidentally, asa method for eliminating electrical noise, the stepped-voltage generatorcircuit 1 a and the stepped-voltage driving circuit 1 c in thestepped-voltage generator 1 were reduced in size to be placed on-boardand then incorporated into the transmitting transducer 2 a, while theamplifier circuit 4 ain the analyzer 4 was reduced in sized to be placedon-board and then incorporated into the receiving transducer. As shownin FIG. 61(a), suppose that the electrical noise and disturbance havenot been eliminated. In this case, although the time indicated by lineX—X of the figure is a theoretical time of generation of a wave of acritical refracted wave, it is difficult to determine the time. That is,standing and non-standing electrical noise and non-standing disturbancehave been generated, thereby causing the wave of the refracted wave tobe buried in these waves.

On the other hand, as shown in FIG. 61(b), suppose that the electricalnoise and disturbance have been eliminated. In this case, various typesof noises are eliminated, thereby making it possible to distinctlyidentify the time of generation of the refracted wave. At this point intime, as a method for eliminating standing and non-standing electricalnoise and disturbance, the aforementioned method was employed forseparating a hardware circuit in the ultrasonic transmitter andreceiving circuits to perform the arithmetic averaging 2,000 times inaccordance with the equation 1.

Incidentally, the time series waves of FIGS. 61(a) and (b) are gained ata center frequency of 120 kHz. FIG. 62 is a graph illustrating a Fourierspectrum with the center frequency being at 120 kHz. In addition, time103.9 μs, which is indicated by the dashed lines in FIGS. 61(a) and (b)is the time for transmitting ultrasonic waves.

In the detection of a reinforcing bar 143 shown in FIG. 60, the angle ofincidence of ultrasonic waves is found to be 42° by applying the soundvelocity in the concrete model _(c)V_(p)=3950 m/s and the sound velocityin the reinforcing bar _(g)V_(p)=5900 m/s to equation 77, describedlater, which is derived from the Snell's law. Thus, the transmissionlength is 67.3 mm×2 in the concrete material 141 and 210 mm in thereinforcing bar 143. Therefore, a theoretical time of reception t_(k) isexpressed by the following equation 74. $\begin{matrix}{t_{k} = {{\frac{67.3 \times 2}{3.95} + \frac{210}{5.9}} = {69.6\quad ({µs})}}} & (74)\end{matrix}$

This matches almost to a measured value 68.3 (μs) determined from172.2-103.9.

FIGS. 63 and 64 are schematic views illustrating time series wavesobtained at each measurement position when electrical noise and the likehave been eliminated. Incidentally, FIG. 64 illustrates the waves withamplitude being made ten times as large as that of FIG. 63. As shown inFIG. 63, at each measurement position, waves having a small amplitudeare generated prior to waves having a large amplitude indicative ofdirect waves. The waves having a small amplitude are derived from wavesof a critical refracted wave via the reinforcing bar 143. In addition,referring to FIG. 64, the waves of a critical refracted wave aregenerated the earliest in time at the positions P23, P28, and P33, whichare located immediately above the reinforcing bar. Furthermore, thewaves have maximum amplitudes. Then, at positions farther away fromthese positions, generation times are delayed and amplitudes arediminished. In the figure, the time of generation of each wave isconnected to that of another by a dotted line. A reinforcing bar isembedded at a measurement position where this dotted curve takes on amaximum value. In addition, the covering thickness d can also becalculated by replacing the time of generation t_(k) at the position ofthe aforementioned maximum value with t₁₁, which is in turn applied tothe equation 80, described later.

As described above, electrical noise and serious disturbances such astraffic noise in measurement environments are eliminated as well as twotransducers are evenly spaced apart in parallel to the direction ofarrangement of reinforcing bars to make measurements and performarithmetic averaging. It is thereby made possible to detect thereinforcing bar even in a concrete material having cracks formed on thesurface thereof. Incidentally, it is necessary to provide the followingsoftware for the ultrasonic detection apparatus.

Now, the contents of the software are described. Here, let y_(A)(t) bethe arithmetic mean wave obtained according to the equation 1 from themeasurement at A1-A2 of FIG. 59 and y_(B)(t) be the arithmetic mean waveobtained according to the equation 1 from the measurement at B1-B2. FIG.67(a) is a schematic view illustrating the arithmetic mean wave y_(A)(t)and (b) is a schematic view illustrating the arithmetic mean wavey_(B)(t).

As shown in FIG. 67(a), in the arithmetic mean wave y_(A)(t), the wave121 of a critical refracted wave via reinforcing bars is blocked by thecracks 125 and the like and thereby only the direct wave 123 isreceived. In contrast, as shown in FIG. 67(b), in the arithmetic meanwave y_(B)(t), absence of cracks for blocking the wave 121 would causethe wave 121 of a critical refracted wave, having an extremely smallamplitude, to be produced prior to the generation of the direct wave123. At this time, the time indicated by the dotted line of FIG. 57(b)shows the time of generation of the wave 121 of a critical refractedwave. In addition, the aforementioned software takes, as a receivedwave, the arithmetic mean wave that has the time, indicated by thedotted line, produced the earliest when measurements are repeated atA1-A2, B1-B2, C1-C2, and so on.

As shown in FIGS. 63 and 64, such measurements made at each measurementposition would make it possible to positively identify the generation ofwaves passing on reinforcing bars as critical refracted waves.

Incidentally, as the distance L becomes larger between theaforementioned transmitting transducer and the receiving transducer, thedifference between the dotted curves, shown in FIG. 64, of a maximum anda minimum value becomes larger, thereby facilitating it to identify theposition of a maximum value, or in other words, the planar position ofpresence of the reinforcing bars.

This tells that the value of L preferably has a large value to someextent. However, the value of L is limited. Too large a value of L wouldcause the aforementioned wave transmitting in the form of a criticalrefracted wave on the reinforcing bar to be reduced in strength, therebymaking it difficult to read the time of generation of the waves of FIGS.63 and 64 to be received by a receiver. It was determined, based on anumber of measurements, that the value of L might be defined by thefollowing equation 75.

L32 300˜500 (mm)  (75)

Incidentally, the aforementioned detection method employing a criticalrefracted wave can also be applied to the concrete having no cracks onthe surface thereof. However, at this time, it should be understood thatthe value of L is defined by the following equation 76. $\begin{matrix}{{( {1 - \frac{{}_{}^{}{}_{}^{}}{5.9}} ) \times L} > {2 \times ( {\frac{1}{\cos \quad \theta} - {\frac{{}_{}^{}{}_{}^{}}{5.9}\tan \quad \theta}} ) \times \overset{\sim}{d}}} & (76)\end{matrix}$

where θ can be determined by the equation 79, described later. Inaddition, numerical value 5.9 in the equation shows thelongitudinal-wave sound velocity (μm/μs) in a reinforcing bar.Furthermore, d is an expectation value of the depth of an embeddedreinforcing bar.

That is, this is applicable under the condition that, when alongitudinal wave is input from the transmitting transducer immediatelydownwards, the time L/_(c)V_(p) for a feeble longitudinal wave producedon the surface of the concrete to reach the receiver is greater than thet₁₁, to be calculated in accordance with equation 80 to be describedlater, indicative of the time of generation of a wave transmitting on areinforcing bar in the form of a critical refracted wave.

The transmission velocity of ultrasonic waves in a concrete material isassumed to be known in conventional detection methods, however, it isnot possible in some cases to measure the transmission velocitybeforehand depending on the concrete structure especially when theconcrete material has significantly deteriorated. However, from theresults shown in FIGS. 63 and 64, it is possible to calculate not onlythe planar position of presence and the depth of an embedded reinforcingbar but also the transmission velocity of ultrasonic waves in theconcrete material. Now, this calculation method is explained below. FIG.65 is a schematic view illustrating the transmission path of refractedwaves at measurement positions P23 and P25. In FIG. 65, the transmissionpath at the measurement position P23 is indicated by a solid line, whilethe transmission path at the measurement position P25 is indicated by adashed line. In addition, d is the covering thickness of the reinforcingbar and b is the distance between the measurement position P23 and themeasurement position P25.

As shown in FIG. 65, the length of the reinforcing bar path at themeasurement position P25 is expressed by the following equation 77,while that at the measurement position P23 is expressed by the followingequation 78.

L−2{square root over (d²+b²)}×tan θ  (77)

 L−2d×tan θ  (78)

On the other hand, the angle of incidence θ can be determined by thefollowing equation 79 in accordance with the Snell's law, where_(s)V_(p) is the longitudinal-wave transmission velocity (5.9 mm/μs) inthe reinforcing bar and _(c)V_(p) is the longitudinal-wave transmissionvelocity (unknown) in the concrete material. $\begin{matrix}{{\sin \quad \theta} = \frac{{}_{}^{}{}_{}^{}}{{}_{}^{}{}_{}^{}}} & (79)\end{matrix}$

Referring to FIG. 65, the diagonal path is a region for ultrasonic wavesto transmit through the concrete material, while the horizontal path isfor those to transmit through the reinforcing bar. Accordingly, lettingt₁₁ be the time of generation of a refracted wave immediately above thereinforcing bar (at the measurement position P23) and t₁₂ be the time ofgeneration at b apart from the position in the horizontal direction (atthe measurement position P25), the following equations 80 and 81 hold.$\begin{matrix}{t_{11} = {{\frac{2d}{{}_{}^{}{}_{}^{}} \times \frac{1}{\cos \quad \theta}} + {\frac{1}{5.9} \times ( {L - {2d \times \tan \quad \theta}} )}}} & (80) \\{t_{12} = {{\frac{2\sqrt{d^{2} + b^{2}}}{{}_{}^{}{}_{}^{}} \times \frac{1}{\cos \quad \theta}} + {\frac{1}{5.9} \times ( {L - {2\sqrt{d^{2} + b^{2}} \times \tan \quad \theta}} )}}} & (81)\end{matrix}$

In addition, since the times of generation t₁₁ and t₁₂ are determinedwith an extremely high accuracy from FIGS. 63 and 64, these values aresubstituted into the equations 80 and 81 to solve the simultaneousequations, thereby making it possible to calculate the two knownquantities or the transmission velocity in the concrete material and thedepth of the embedded reinforcing bar. From FIG. 64, sincet₁₁=172.2−103.9=68.3 (μs), t₁₂=182−103.9=78.1 (μs), b=60 (mm), and L=300(mm), the equations 80 and 81 give that d=49.5 (mm) and _(c)V_(p)=4.0(mm/μs). It can be said that the transmission velocity is calculatedwithin 2% error with respect to the actual transmission velocity ofultrasonic waves. As described above, it is possible to determine notonly the depth of the embedded reinforcing bar but also the transmissionvelocity of ultrasonic waves in the concrete material.

Incidentally, the aforementioned value of b may be determined by thefollowing equation 82 using the planar minimum distance (the value of Sin FIG. 60(b) between the position at which the aforementioned t₁₁ hasbeen obtained and the reinforcing bar embedded in parallel.$\begin{matrix}{b \leq {\frac{1}{2}S}} & (82)\end{matrix}$

Industrial Applicability

As described above, according to the present invention, provided is anarithmetic averaging device for performing arithmetic averaging, everytime an ultrasonic wave is received, on the ultrasonic wave and theultrasonic waves that have been received until then, thereby making itpossible to gain, by the arithmetic averaging, only such waves that havenot been substantially changed in their phase. Accordingly, measurementscarried out under the conditions which cause substantially no change inphase of a desired wave would make it possible to detect, with highaccuracy, the thickness of a concrete material narrow in width and thickin thickness, the thickness of a reinforcing bar covering, and thediameter thereof, and the depth of cracks. Furthermore, the arithmeticaveraging device performs directly the arithmetic averaging, therebyobviating the need for purpose-oriented software and the like and makingit possible to perform processing at high speeds.

Furthermore, according to the present invention, while a transmittingtransducer for transmitting ultrasonic waves and a receiving transducerfor receiving ultrasonic waves are moved within a predetermined regionon the surface of the material being detected, transmissions andreceptions of ultrasonic waves are carried out a plurality of times toperform arithmetic averaging, every time an ultrasonic wave is received,on the ultrasonic wave and the ultrasonic waves that have been receiveduntil then. This makes it possible to vanish unnecessary received wavesand thereby gain only the desired received wave. Furthermore, arithmeticaveraging is performed on the arithmetic means for each distance betweentransducers, thereby making it possible to carry out detection with highaccuracy even under harsh conditions.

What is claimed is:
 1. An ultrasonic detection apparatus, including areceiving transducer configured to receive a plurality of ultrasonicwaves transmitted by a transmitting transducer, comprising: anarithmetic averaging device configured to perform an arithmeticaveraging process a plurality of times per one detection, each of saidplurality of times performed in response to one of said plurality ofultrasonic waves being received by said receiving-transducer, saidarithmetic averaging process includes averaging said one of saidplurality of ultrasonic waves and at least one prior received ultrasonicwave of the plurality of ultrasonic waves so as to produce a set ofaveraged ultrasonic waves; and an extractor configured to extract anextracted ultrasonic wave from said set of averaged ultrasonic waves,said extracted ultrasonic wave having a predetermined frequency as acenter frequency, wherein said predetermined frequency is given by((n±(½)×(10⁶×v/ΔL)(Hz), where ΔL is a variation in a distance betweensaid transmitting transducer and said receiving transducer, v is anultrasonic wave transmission velocity of a material being detected, andn is a natural number.
 2. The ultrasonic detection apparatus accordingto claim 1, further comprising: a transducer-distance adjusting deviceconfigured to adjust said distance between said transmitting transducerand said receiving transducer by a predetermined value every time apredetermined number of arithmetic averaging operations is performed. 3.The ultrasonic detection apparatus according to claim 2, furthercomprising: a transmitting circuit configured to output an electricalsignal to said transmitting transducer; a receiving circuit provided ina housing different from one for said transmitting circuit andconfigured to receive an electrical signal from said receivingtransducer; and a received-wave determination device connected to saidreceiving circuit and configured to determine an earliest ultrasonicwave when predetermined ultrasonic waves are included in a plurality ofultrasonic waves received at a measurement point, said earliestultrasonic wave having the earliest time of generation among saidpredetermined ultrasonic waves received wave at said measurement point.4. The ultrasonic detection apparatus according to claim 2, wherein saidtransducer-distance varying device comprises: a plurality oftransmitting oscillators evenly spaced apart from said receivingtransducer and disposed within said transmitting transducer.
 5. Theultrasonic detection apparatus according to claim 2, wherein saidtransducer-distance adjusting device comprises: a plurality of receivingoscillators evenly spaced apart from said transmitting transducer anddisposed within said receiving transducer.
 6. The ultrasonic detectionapparatus according to claim 1, wherein said arithmetic averaging deviceis configured to perform arithmetic averaging 1,000 times or more perone detection.
 7. The ultrasonic detection apparatus according to claim1, wherein said distance between said transmitting transducer and saidreceiving transducer is adjustable.
 8. An ultrasonic detectionapparatus, including a receiving transducer configured to receive aplurality of ultrasonic waves transmitted by a transmitting transducer,comprising: an arithmetic averaging device configured to perform anarithmetic averaging process a plurality of times per one detection,each of said plurality of times performed in response to one of saidplurality of ultrasonic waves being received by said receivingtransducer, said arithmetic averaging process includes averaging saidone of said plurality of ultrasonic waves and at least one priorreceived ultrasonic wave of the plurality of ultrasonic waves, said oneof said plurality of ultrasonic waves being formed by applying a stepfunction voltage to an oscillator, wherein said predetermined frequencyis given by ((n±(½)×(10⁶×v/ΔL)(Hz), where ΔL is a variation in adistance between said transmitting transducer and said receivingtransducer, v is an ultrasonic wave transmission velocity of a materialbeing detected, and n is a natural number.
 9. An ultrasonic detectionapparatus, including a receiving transducer configured to receive aplurality of ultrasonic waves transmitted by a transmitting transducer,comprising: an arithmetic averaging device configured to perform anarithmetic averaging process a plurality of times per one detection,each of said plurality of times performed in response to one of saidplurality of ultrasonic waves being received by said receivingtransducer, said arithmetic averaging process includes averaging saidone of said plurality of ultrasonic waves and at least one priorreceived ultrasonic wave of the plurality of ultrasonic waves, said oneof said plurality of ultrasonic waves being formed by applying a stepfunction voltage to an oscillator; and an extractor configured toextract an extracted ultrasonic wave from said set of averagedultrasonic waves, said extracted ultrasonic wave having a predeterminedfrequency as a center frequency, wherein said predetermined frequency isgiven by ((n±(½)×(10⁶×v/ΔL)(Hz), where ΔL is a variation in a distancebetween said transmitting transducer and said receiving transducer, v isan ultrasonic wave transmission velocity of a material being detected,and n is a natural number.
 10. A method for detecting an ultrasonicwave, comprising the steps of: transmitting a plurality of ultrasonicwaves with a transmitting transducer; receiving said plurality ofultrasonic waves with a receiving transducer; moving said transmittingtransducer and said receiving transducer within a predetermined regionon a surface of a material being detected; performing arithmeticaveraging every time on a received ultrasonic wave and on a plurality ofpreviously received ultrasonic waves so as to produce a set of averagedultrasonic waves; and extracting an extracted ultrasonic wave having apredetermined frequency as a center frequency from said set of averagedultrasonic waves, wherein said predetermined frequency is given by((n±(½)×(10⁶×v/ΔL)(Hz), where ΔL is a variation in a distance betweensaid transmitting transducer and said receiving transducer, v is anultrasonic wave transmission velocity of a material being detected, andn is a natural number.
 11. The method for detecting an ultrasonic waveaccording to claim 10, wherein said moving step comprises: moving saidreceiving transducer within a first region, said first region set to beoutside a predetermined transmitting transducer movement region.
 12. Themethod for detecting an ultrasonic wave according to claim 10, whereinsaid moving step comprises: moving said transmitting transducer and saidreceiving transducer immediately above a target being detected in saidmaterial being detected.
 13. The method for detecting an ultrasonic waveaccording to claim 10, wherein said ultrasonic wave is transmitted andreceived 1,000 times or more.
 14. The method for detecting an ultrasonicwave according to claim 10, wherein said transmitting step comprises:applying a step function voltage to an oscillator.
 15. A method fordetecting an ultrasonic wave comprising the steps of: transmitting aplurality of ultrasonic waves with a transmitting transducer; receivingsaid plurality of ultrasonic waves with a receiving transducer, saidreceiving transducer is moved within a predetermined region on a surfaceof a material being detected; performing arithmetic averaging every timeon a received ultrasonic wave and on a plurality of previously receivedultrasonic waves so as to produce a set of averaged ultrasonic waves;and extracting an ultrasonic wave having a predetermined frequency as acenter frequency from said set of averaged ultrasonic waves, whereinsaid predetermined frequency is given by ((n±(½))×(10 ⁶×v/ΔL)(Hz), whereΔL is a variation in a distance between said transmitting transducer andsaid receiving transducer, v is an ultrasonic wave transmission velocityof a material being detected, and n is a natural number.
 16. A methodfor detecting an ultrasonic wave comprising the steps of: repeating apredetermined number of times the steps of: transmitting a plurality ofultrasonic waves with a plurality of times a transmitting transducer;receiving said plurality of ultrasonic waves with a receivingtransducer, said receiving transducer within a predetermined region on asurface of a material being detected; performing arithmetic averagingevery time on a received ultrasonic wave and on a plurality ofpreviously received ultrasonic waves so as to produce a set of averagedultrasonic waves; varying a distance between said transmittingtransducer and said receiving transducer by a predetermined amount;determining an arithmetic mean of ultrasonic waves obtained as resultsof said arithmetic averaging; and extracting an ultrasonic wave having apredetermined frequency as a center frequency from said set of averagedultrasonic waves, wherein said predetermined frequency is given by((n±(½))×(10 ⁶×v/ΔL))(Hz), where ΔL is a variation in a distance betweensaid transmitting transducer and said receiving transducer, v is anultrasonic wave transmission velocity of a material being detected, andn is a natural number.
 17. A method for detecting an ultrasonic wavecomprising the steps of iteratively: transmitting a plurality ofultrasonic waves with a transmitting transducer; receiving saidplurality of ultrasonic waves with a receiving transducer, saidreceiving transducer within a predetermined region on a surface of amaterial being detected; performing arithmetic averaging every time on areceived ultrasonic wave and on a plurality of previously receivedultrasonic waves so as to produce a set of averaged ultrasonic waves;extracting an ultrasonic wave having a predetermined frequency as acenter frequency from said set of averaged ultrasonic waves; and movingsaid transmitting transducer and said receiving transducer on a surfaceof a material being detected, wherein said predetermined frequency isgiven by ((n±(½))×)10⁶×v/ΔL))(Hz), where ΔL is a variation in a distancebetween said transmitting transducer and said receiving transducer, v isan ultrasonic wave transmission velocity of a material being detected,and n is a natural number.
 18. The method for detecting an ultrasonicwave according to claim 17, further comprising: calculating atransmission velocity of an ultrasonic wave in said material beingdetected in association with a time of generation of a predeterminedwave appearing in a result, obtained at a different position, of saidarithmetic averaging.